This may sound similar to a certain feature from a certain other MMO.

Summary -

An inventory, like the Fashion Accessories menu, accessible through the Character tab containing a wide variety of infinite-use glamour prism items that each grant a unique effect to the user. Prisms would only be usable outside of combat, instanced duties, etc., as per usual. For this example, a selection of prisms could, for a temporary duration, bestow transformative effects upon the user, such as granting them the appearance of various non-player characters, monsters, animals, minions, etcetera.

These transformations may or may not also grant access to various abilities associated with their respective visage, such as the action 'Bad Breath' being usable to a player glamoured as a Morbol, or '1000 Needles' to one glamoured as a Cactuar. Actions granted when transformed would be purely cosmetic and have no effect, of course. Other prisms may imitate the appearance of performing various actions that, normally, would only be usable during specific quests or duties, such as transporting large objects, or any number of duty-actions one might find in raids. Another prism may allow the user to take on the appearance of another targeted player.

Given the scope of possibility, these prisms could be distributed throughout the game in any number of ways, be they purchasable from vendors, inside coffers in dungeons and raids, as crafted items, PvP rewards, quest rewards, in the Gold Saucer, from achievements, etc.. Upon acquisition of a unique prism, the player may use it, registering it to their character in the same way one would with fashion accessories, orchestrion rolls, or emotes. That prism may then be used an infinite number of times from the Prism inventory.

The idea of this came about when I considered the abundance of events throughout the game that either transform the player into another form, or allow certain actions only usable under those limited, specific scenarios. Various trials and raids will have moments where the player is transformed into a chicken, or a snowman, or a zombie, for example. The 2022 All Saints' Wake event introduced an area where players could take on the visage of a huge number of characters throughout the story. It was very memorable.

I think giving players the ability to do this freely would be very fun. Much like glamour, emotes, or fashion accessories, this system would give players another way to just have fun, and provide a wider scope of items to collect, craft, and trade, while offering unique moments in ways that emotes or fashion accessories currently cannot. I'm sure roleplaying groups in particular would greatly appreciate a feature like this.