A few random thoughts I've had here and there (non-comprehensive / short quips):
- Chocobo revamp that includes hot and cold gameplay (+chocograph system, edit: may also interact/intertwine with our current chest system but wasn't the intent, chocographs would be different than normal chests).
- Likely related to the above, Archelogy profession / content concept. Opportunity to learn about the world while also traversing all over it, while rewards could also be interesting in that they come from specific histories (like you might learn of / gain more info of a particular long ago town and then earn some decorations, or a great warrior and then get a recreation of their armor at the end of the journey). Repeat activities being considered (like the hot and cold could make a nice way to level your chocobo), but seeing as the game space is so large now they can have a lot of unique tasks such that you get to enjoy many parts of the game world with changing scenery, instead of being stuck / told to farm in one area only because the rest are useless to your level).
- Museum / Codex content, also likely to relate to the above, but akin to a bestiary except it can be about areas, objects, monsters, and people, yet also doesn't have to be 'everything' (if the content is massively popular they could expand it, if it's decently enjoy-able they can keep up with the most interesting concepts only, and if it's dead on arrival then they didn't put too much time into the system).
- Mega-leve which act as a multi-zone multi-step madlib sort of quest that provides important rewards (like if you don't get to play often you may choose end game stones, but if you did you'd pick some other major currency / reward- for the time cost these would always be worth it but you'd only be able to do at max like 1 a week).
- Monster Hunter revamp to the hunt system, with a hunt log (UI) rather than key items, besides trying to make them more active (which SE has done quite a bit over expansions already) also adding a lot of content related around it. These would be options to progression and absolutely not requirements (valid option, but not the only one). Add some new C rank concept (just general monsters, not special names), more aggressive / faster AI, make some monsters of this family non-aggressive where lore make sense (like I don't see why all deer need to be aggressive in ShB lol). C ranks while not being particularly hard for a fresh cap player (say fresh 80 in a level 70 zone) would still aggro the player and still take more than a few hits (and a level 75 fighting a level 75 c rank would likely have to use their whole kit, but perhaps get bonus exp to make it worth while).
- Going off that on a solo level I think adding unique items to B ranks (and them scaling to your level, at least within 10 levels), gear that is very strong during your time leveling but wont replace any end game pieces, perhaps with special effects that only work when you're solo (like a dragoon spear might replace thrust with fire breath after x jumps). Naturally a B rank scaled down would require your whole kit, knowledge of markers (look here, dodge there, etc), and at least moderate skill (don't need to be a pro, but if you just stand about and face roll the keyboard you'd for sure lose).
- Further on solo but not just for solo is that like S ranks that have special spawn rules there may be some new hunts that you can spawn but will be /just/ for you/your group. This could be part of the leve system (which at the time of mega-leve being added may get a rework itself so perhaps they're a bit more rewarding but less spammy). Not all leve have to be ending in a hunt related task, but their balance and reward being less in and out - like they might take 5-10+ minutes to complete all the tasks rather than 2-4 but also come with double the reward.
- Personally think there would also be some value in making hunts in general not as focused for 400 people trains but that they reward those groups that find them first just a little bit more (like 1 extra B tier drop, A tier drops just dropping as normal), then when a monster dies they leave essence that deteriorates over time that can be used as part of a purification / sending (FFX) like ritual that allows other players to engage in a sort of burning circle content with that hunt, the older the death the easier it is but with less drops at each weaker tier (can manually choose to make it weaker), in this way more types of groups can enjoy the content, the monsters can be a bit more tuned to specific numbers, and if you missed the train doesn't really mean you missed the content, while at the same time this change would allow for monster hunter like crafting update to not be so annoying if you missed a specific monster when it was fresh (yet being the first / the 'hunting' party is kind of fun / a race, but if you lose the race you're not really ruined). Naturally there will need to be resetting timers or tokens on the sending ritual so you can't simply find the body of a hunt and repeat it 50 times in a row (hopefully a system that is relatively easy to deal with, yet keeps a balance so you don't find Leaping Lizzy's essence and kill him non-stop over and over in a row until he has no use to you anymore, want to keep the journey vibe of the system). The decay of the hunt's essence can also be used to determine when the monster will respawn (roughly).
- Add purchase unlocks to beast tribes, so like now when you get a rank you unlock gear and some new visuals. Perhaps also you might see for 20 dwarf tokens they create a special water siphon in a newly unlocked cave, this unlocks new items from your shop but also the cave has a node for miners.
- Personally crafted gear that uses exclusively open world resources (may include beast tribes), gear that might use many gatherers and crafters but is a solid choice in casual end game content and comes with an upgrade option (which can use more instanced content resources) to make it a really good choice. Because it can't be sold / traded each player will have to go on a world wide adventure to make it. May save resources by having it look similar to the crafted sell-able gear (needs some differences ideally). If wanting to avoid BiS issues might ensure the stat allocations tend to have a very slightly less amount of damage (still comparable) and might have a bit more on QoL / survive-ability (the allocations might break standard allocation rules to follow this goal, such that the upgraded form is acceptable in pretty much all content but you're not going to have someone told they need to farm 20 hours and level all crafters and gathers or they can't play in their savage group). Another idea might be that you could pay an npc to make the items for you, to give players who don't want to level all the crafts an 'out' (in such a case then the recipes shouldn't be too hard to make or if you use the npc it requires some extra resources, so a player risking the ingredients to self craft is justified to do so). While you can save money and use the crafter's sell-able gear as a visual I think it would be fun to have the gear come in tiers, like a relic concept, and each visually theme'd a bit like a warrior living off the land but include quality craftmanship and the theme of that expansion (so if you did this in Stormblood you'd have some pieces of armor that you could tell what area they came from based on the materials used but at the same time it wouldn't look shoddy-made and would have an Asian theme appearance).
- New cosmetic / roleplay merit system that uses exp from when you're cap but also from certain open world content like FATEs (their bonus exp system may also translate into a special extra bonus to merit), new leves, hunts, etc, where you can purchase like a sphere grid or license board. You could put bard instruments in here, choreography dance system, dragoon jump ability that allows them to temporarily jump much higher (pathing will take in account if you have unlocked flying or not), reduced / no fall damage on dragoon (nodes can have tiers), certain teleport locations on mages (which can be upgraded so you don't need to switch to that job to take advantage of it), gear that wouldn't normally get made for a job type like Holy Breastplate for White Mage, mounts could make a logical appearance here (like Ninja could unlock their frog as a mount, again you might suggest the first purchase creates a special ability just for Ninja, like Dragoon with jump, that has some special rules like you can use it in combat when not in party content, but if you upgrade the skill you unlock the non-ability mount from any job can use), or being able to unlock the shadowy aura effect from Dark Knight's trailer rather than the red aura for darkside. Etc. You'd never have a situation in party content where you're not invited because you didn't have x y z merits, but beyond simple cosmetics there may be roleplay concepts that add some vibes to the knowledge of being a dragoon (like no fall damage on dragoon or a special ability you can use to be able to hulk/dragoon jump simply with your space bar, given that you're not in a party and you're not in town or something lol). The points gained from exp might not be a simple 1:1 system but with intent that as you enjoy the game you might have unlocked 1 to 3 neat merit nodes by like 50 (say a dragoon at 40 may have been able to purchase the first tier of reduced fall damage), but once you hit cap while you'll be gaining more points more consistently you wont instantly fill out the board. Of course at cap the roulettes first time bonus can help with the merits but open world would be one of the only places that you could get exp to your merit board even if you're not capped yet, making it so as you're wandering around the world doing other activities those open world tasks seem a bit more interesting (but wont demand you sit there and farm them because you also know some of the more expensive / later merit toys will be opened up / easier to purchase later).
Still want to think about adding airships, thoughts of flight, etc, but list is already pointing in too many directions lol.
OF course in the shortest words possible what Edax said is it lol. Does the content offer something of value?