First of all, I don't see how a series where each and every entry is unique and has nothing to do with the previous ones (apart from a few recurring names) could be "well past done". They don't even share the same gameplay or combat system.

And also, Square has "tried something new" pretty much all the time. Triangle Strategy, Live a Live, Nier, Kingdom Hearts, Star Ocean, SaGa, Mana series (with an upcoming title next year), Dragon Quest, Octopath Traveler... I'm just listing a few, but I could go on. It's not even a thing they suddenly decided to do. Even a few gens back, I remember playing Star Ocean, Last Remnant, ~~Resonance of Fate~~ and other lesser known titles. If you want to go back to the PS1 days, Vagrant Story and Threads of Fate come to mind, but there are surely a lot more I'm currently forgetting. EDIT: Xenogears too! I don't know how I could forget that one.