In base form I think a mix of instant and short cast times spells (not black mage duration, a little samurai ish feeling) with again a sort of Bayonetta visual, so perhaps some of these skills are mid range and have a sort of physicality sense of coming from you (not long cast or as out of no where (fire 4) like black mage). If you stayed in base form only you couldn't compete damage wise with other jobs though (this is a transformation job! lol), it just offers some functionality and can be expanded on in the mage form (your damage comes from properly utilizing your forms).
For the transformations I don't really want to go into depth on potency and such because I believe SE will do what's best there but I might try to help suggest some transformations and gimmicks.
The first three transformations would be core transformations, including a melee damage focus, melee bruiser (solo tank), and ranged damage focus. Outside of that there could also be augmented self/mage form and transportation transformations like Druid in WoW has, which are actually pretty neat because they can be cast in combat.. (could restrict use when in dungeons though if that's a concern).
May also consider ultimate temporary forms like Demi-Bahamut with Summoner (temporary mechanics) and or upgraded versions over time (so expansions have something special to offer as the job upgrades, WoW has some "glyphs" that can alter the visual appearance of some stuff).
So you'd have at least a few forms:
1 Bruiser (semi tank)
1 Damage Dealing Melee
1 Damage Dealing Ranged
x Damage Dealing mid range void form?*
1-3 transportation forms, you could have one for water, ground, and air, or SE could bind them all into one. The definitely "coolest" option would be that there are three and are automatically transformed between as it makes sense (enter water from air = automatic change to form, and not just the same mount visual).
0-x ultimate, can be part of the job mechanic that for short periods you get a much larger wild berserk looking form. May be future content.
In WoW these forms allow the druid to play different roles, when they spec for it, such that a druid could truely tank or damage deal or heal - obviously FFXIV doesn't work like that with the talent tree not being a thing here. So what I think you could see is phasing between forms each having a unique mechanical style of gameplay such that you might feel as someone given many forms to combat. Preferably rather than the forms having timed lives I'd like to see mechanics that encourage shifting, such that while you could stay in one form after a certain period of time or actions you'll get a bonus in the next (as well as the mechanics of a form might be helpful for your content, like in tight quarters or needing speed to dodge aoes). This could be as simple as each time you use an ultimate in a form you can't use the ultimate build up skill in that specific form again until you've been in two other forms first, but there are no other negatives or bonuses (this is still a BIG deal, but if you needed that extra second for raid utlity or timing you get it). Ultimate build up skill/ideas are listed below, just concepts at least.
Each form could have a few gimmicks not only mechanically but also in terms of what they offer:
Bruiser - the "tank" form, here I'd suggest no mobility skills but a sort of mechanic to shrug off moderate damage on interval, such that you might be able to help pick up stray trash in a group dungeon but you definitely wouldn't be able to hold a boss. Or in other words you could hold a boss about as well as titan egi could (and can't take tank busters). Because this form is on a damage dealer I'd suggest the damage from skills in this form are actually enhanced when taking damage, meaning when solo this form is definitely viable even if you over gear the monsters (it's not just a "help me I'm not prepared/strong enough" like titan might be seen as, but it wont dethrone a tank ability to take damage or mean you can soak tank busters).
While used more in solo play this form could help if your healer is stressed and you know unavoidable raid damage is coming, your secondary tank died and your main tank needs help with adds (for a short while you could hold out), it may also help keep rotation bonuses (bonuses from changing forms, rather than sitting in one mode only if another form is one you can't use optimally for a specific fight mechanic).
Melee damage dealer - I think an interesting gimmick could be every attack skill acts as a short distance gap closer, giving a sort of ravaging clawing demon effect (enemies cannot escape you, but all your mobility is towards the enemy - going away from the fight is not a skill it has). I also think a skill that moves you to the closest flank or opposite flank if you're on one already, combined with positional skills would be neat. So you'd be clawing up and moving very effectively from side to side and rear.
When you leave an area with a properly built up combo jump you could lightly highlight that section on your screen only as a void wound (like left flank has a light effect on the circle) and once both flanks have been inflicted you could use a combo on the rear that opens the entire back side to any positional for the raid (True North applied to the backside, all weaponskills completed during this time have a slight potency boost). During an open backside I think greatly lowering personal cooldowns could be fun, so you become a nightmare of flurry attacks - once completing this though you'll likely want to switch to keep rotation bonuses (remember I suggested there should be a mechanic that rewards you for switching after a certain amount of time or actions, so good players are rewarded with diversity).
Ranged damage dealer - Multi armed winged demon - Glides swiftly and has a few peppering attacks, like one might think of with bard. Mechanic with conjuring bolts in hand automatically as a sort of ammo system (multi-armed), with movement and certain skills encouraging their return rate. What would encourage a player from firing them all off at once or waiting would be a few skills that do things like greatly increase recharge rate (or even make another pair of arms) and skills that for a short time increase/combine bolt damage (for example the increased damage buff could be a shorter cooldown, such that it would line up once with multi-arms ultimate skill and then another time without such that you might hold off on firing everything at once if the oGCD is almost ready). Comes with a few dash left and right attacks (could be one skill that fires off a void bolt and dashes in the direction you're currently moving).
Finally I think what could mix in with the high mobility is a sort void pylon mechanic, after a certain successful criteria is met (like number of fired projectiles), you can place a void pylon. The ultimate skill for this form would be to stretch out the arms and raise up each pylon into projectiles - what might visually feel like very large Red Mage Fleche skill or large lightning bolt toss from Dark Souls but void themed (fleche is the blue sword oGCD skill red mage has). Here however the amount of damage would involve the quantity of pylons placed before activation and their distance from each other (there would be a max distance, you don't need to go to the four corners of the map but they would optimally not be right next to each other). Around the time of completing this ultimate you'd be near wanting to change forms for bonus.
A raid utility could be from the void pylons providing something to the team, meaning your placement is quite thoughtful (they need to be apart from each other but also in key places for allies). This utility should ideally be different than what mage form or melee form offer in either an enemy debuff or team member buff (in general speak mage form offers light restoration utility and melee is damage for at least weaponskill based jobs).
Mage form - If SE really wanted to go all out I believe you could take the base form of the job with it's skill and "enhance it" with mage form. To keep the high mobility of the job in this form when a spell is cast the spell is cast from a sphere of void energy (created via another mechanic) with a longer cast time but higher potency (but it's not being cast by you, so you can move around). For balance I believe a mechanic could be put into place to build up how many void spheres you have out at once and the cast speed, when you're out of void spheres (because they're casting) your spells become instant but less potency than what is coming out of the spheres. Mechanic in this job could be casting different spells and creating more powerful variants by grabbing a spell and combining it with another.
For example lets say you cast a Shadethrust like ability (Scathach), you might grab another to slightly increase the radius (two barrel attack) but it'd also slightly increase potency and more-ever cause the second one to cast off instantly meaning more damage in the long run (rather than waiting for two separate off timed casts you accelerate it into one). Combing would have a cooldown. Alternatively you cast two shadethrust, void spike (picture Warcraft 3 Flamestrike but with FFXIV graphics), cast a room wide potency enhancement spell (increases ally skill damage a little but more specifically to your orbs), and use fast cast to get off your combine magics spell off before the first shadethrust vanishes causing an almost limit break like visual on the boss (and very heavy damage). So the ultimate from this job (as the other ones sort of have a "final moment of glory" before you should probably switch) would be building up to maximum void spheres possible and then combining it.
The mechanic to gain more spheres could perhaps have to do with being able to cast a spell in a target radius, causing your sphere to rush to that area and allow you activate one of your sphere emanating skills. Each emanation skill would cancel the other, but you could choose between light healing, light damage, or light resource restoration - the effectiveness of the placement and choice rewarding you to earn more spheres (so lightly healing 6 allies obviously worth more than dealing light damage to the boss, but if it was a trash group then light damage worth more than light healing).
This job's mobility wouldn't match the other two in speed but also consider an orb swap skill (the orbs show up in your list like a chocobo/egi ally might for easy targeting and easy to know what they're casting). Other fun mechanics could include chucking your active spheres at allies or enemies not only having an immediate aoe affect but also improving positional for subsequent spells. Pulse skill that causes all active spheres to double tap their current emanation effect and fire at target, they would also lock onto the target with their currently casting spell IF their current target had died (meaning the spell wont be cast at a corpse, you can redirect spells if you want - technically some line attack spells might still be okay to hit a corpse). There are a lot of ideas and perhaps some should be saved for a possible geomancer like job but just to end with one more, a chariot skill that causes the target sphere to allow a player to jump on the sphere and be whisked away to your location with a % mp and hp heal in the process (jump being the consent action, so its not on accident - the mp restoration being the main nice part and the heal being acceptable).
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Sort of an aside but later in the character's development I think the transformations happening without disrupting movement (such that stringing a form into the next form feels like slicing melted butter) would be really cool, sort of a story point of your character reaching control of their ability and making it feel great to slide between forms.
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