I think I really cool addition to FFXIV would be some way to have different (cosmetic only) appearances for your summons, sort of like how right now you can get different looking (but functionally identical) mounts or how in World of Warcraft it's possible to unlock different versions of the spell Polymorph to turn enemies into things other than the default sheep.

I like cute things so I'm a big fan of carbuncle and the scholar's fairy summons but I don't really like the summoner primals. I think it'd be cool if there was, for example, a way to make the egi look like various primal-themed carbuncles or fairies or something entirely new. Likewise I'm sure some people would like the opportunity to have a more exciting looking scholar pet. Cosmetic customization is an important part of self-expression in MMOs (Race, customizable appearance, gear, minions, mounts, and so on) and arcanist pets seem like a great opportunity to expand customization to.