So like you attune to a primal empower and building but at a certain point you /must/ switch, perhaps also when leaving an attunement you hold that power shortly so proper play is not only switching up the summons but doing it with understanding of order and interaction and the content being performed. Summon Ramuh Egi, you enter Levin stance, apply your "bio" DoT (Electrostatic, smaller potency than other versions but increases potency per nearby enemy with same DoT - perhaps with a Ramuh Egi interaction), Imbibe Essence- Ramuh (builds a stack) and now Ramuh Egi brings forth two Grey Arbiters that cast Levin Bolt with you and increase judgment bolt, Levin Bolt, somewhere around here building up too many Levin stacks and so you decide to cast Judgment and changes them into Depleted Levin. Next summon is free and half cast time, with a Levin specific small buff on the depleted stacks. Buffs could be interesting or general, like 10% spell speed, DoTs have a chance to tick twice, but I think trying to theme them would be fun like from Shiva, Shatter - all spells add 5 stacks of .5% MS reduction for 5 seconds and .5% more damage from your own spells stacking to 100 and no resistance building (DoT ticks add 1 stack), immune enemies suffer 7% damage on impact instead (immune is like bosses, so flat damage increase for bosses but with normal enemies you'd get this stacking damage - mix that with an Electrostatic and could get some super sexy AoE power or single target on focus burn trash like mini bosses).
Should consider spells that can use Depleted Stacks (depleted uranium lol) but keep in mind they also serve as a timer till you can safely return to Levin stance and add buffs. Maybe one of the depleted stack consuming abilities is your DWT ability, removes Depleted Trails returning mana and if three different primal energies were removed places you within DWT (longer/enhanced/more stacks if more than three different primals). Point being it can be used to emergency remove say Depleted Titan Cores so you can return to Titan stance full bore (if you /needed/ Titan's specific spell benefits; however, normally you'd wait till you can at least enter DWT before casting away your depleted stacks).
At this point the system would /generally/ demote using a single primal too much and more specifically encourage you use many primal egis (no Gaurda/Ifrit 24/7), but there would also be some niche opportunities for each summon (give each summon some unique points like WoW, but preferably better.. haha) with a oh crap I made a mistake/oh crap I'm a god now cooldown ability (consuming depleted trails). There would be quite a bit of thought on the situation you have, the enemy make up, and the boss's stages (if a boss has a middle stage with adds you'd best accommodate your primal cycle so you could use the best egi for that moment).
In order to better accommodate the summoner during trash, first just as a dungeon design comment I'd like to see more mini bosses and "staged" fights where they have their own mechanics, less just random trash and more mini-burning circles and bosses, but added to that the depleted stack consuming ability could have it's cooldown reduced if it consumes less than 3 primal energies (allowing for you to come back to specific Egi more often when you're not in longer fights, but you'd still be looking at using at least two different egi per fight and given that each BCN has different scenarios you'd probably use different Egi each time too, like heavy antimage moments, and stuff like that).
For example of depleted buffs carrying Depleted Titan Cores could give you 10% more max HP and your Egi 25% (and the enmity table is now shared between all egi while the depleted cores are up). There would need to be a toggle on titan though, to make his enmity into extra damage but remove it (so Titan can /untank stance and therefore you can use him in more scenarios).
To better explain why you might consume stacks early even though it didnt get you your DWT is lets say everything is going down the drain but Titan's skills have things like team buffs that you could use to help, like one of the Titan abilities you unlock while building energies like Soul Gaol, an instant cast spell with decent cooldown: On enemies a strong CC (on non bosses) that will also bind their spirit into the prison dealing damage at the end of the timer as it changes into a titan bomb (attacking the gaol accelerates the timer and adds potency, if the enemy is not present when the timer is up it will wait at least 15 seconds before exploding (high single target, lower aoe) or can be activated again to explode then and there for aoe damage (can still attack it while the charged Soul Gaol waits for the boss to return but at reduced effectiveness, for balance reasons), this also could be a way to attack the boss when they're off screen allowing summoner DoTs to better shine in high phase fights); however, if you cast it on a downed ally it will glow and begin to pulse/shock them into a revived state and allies can attack it to shorten the duration, if a healer casts raise on a gaol'd target the raise MP is refunded based on time remaining (minimum refund at 40%) and Gaol duration finished. With a few other abilities like this Titan could be a strong moment for you, as you effectively raise two players - of course you can't replace the healer as your heals are still crap (or at least "usually") but this would be a moment where you'd choose to not get to DWT and just save the party from a wipe.
Punishment for going too deep into stance could be a number of things, and with the Titan egi hp buff I'm less concerned about player's trying to solo and being unable to because the fight is longer than they can keep Titan out before having to switch (if they must switch they will bring out a much higher HP egi, and share the enmity table - and thus should be fine in any solo situation). Perhaps to make transitions smoother rather than summoning out and in an egi rather the egi itself morphs (would mean shared enmity table is a default trait, could make it 25% more hp and ignores knockback effects then).
Unfortunately I don't see them able to do this past the base elemental primals (the six main elemental primals), and perhaps elder primals as part of the depleted stack consuming stance. No Sephirot, at least in mechanics - could have an egi glamour. I think you could reasonably come up with six different styles. I think Duelle already listed six different niche scenarios, so again I don't think six different egi is an issue but more and more would be harder and more bloat. Lore into why you can only summon the base elemental primals I think is possible, like how we're collecting the crystals in the story that could be part of it (the elements + light and dark for the elder primals).
It would become interesting to see how the best summoners order their Egi though as well as reactions to player errors/RNG of fights. Obviously a specific summon would be "the best" of a scenario but the system would still make you choose at least two others to use as well, having all 6 depleted cores could mean something like instead of DWT it's like Dreadwyrm Pact, Falcon Punch the boss with your Bahamut Claws for a little bit lol. Important thing here to note is that DWT requires you to play with your Egi and other primals to obtain, you'll not be the Dragon Priest as Duelle funnily announced the job lol.
Example of how you may plan your egi order: You may choose to bring in high single target at the start of a fight with one boss, the boss summons adds you go high AoE, boss prepares a big ulti and so you get yourself that Titan shield and right before it finishes you switch out to get the HP buff too, then you're in 110% burn mode and consume your 3.5 stacks to go into DWT as the boss has a DPS check on. Meanwhile other fights it may be helpful to debuff/cc them - especially changing order in normal dungeon mode, or slightly buff your allies, raise, whatever - also of course meaning if you have multiple summoners there would be benefit for at least some variation in their own rotations based on them being together (good teams wouldn't buff/debuff fight).
Although all in all I feel the DWT doesn't have to be removed but only tweaked a bit to unwind the mechanics to give more room for other features and mechanics and breath a bit more into the Egis (again my not very unique suggestion was just to make it like Warrior stacks/consumable Monk Lightning which give you a bit more room while not changing too much - could still have your "Fel Cleave" / "Ahk" mechanics). While I'd like to see SMN change I also know a lot of people actually like how it is currently, like exactly how it is lol.. The changes in SB cannot come soon enough, really hope they can blow it out of the park - at least on a majority.