Creating a new menu window (concept)

Constructing the new window is actually *relatively* simple, especially if you're working on the optimised/reassemblable C3 -- I've been working on a couple of original windows for Tensei. (... which will come out of development hell eventually, if I can ever stop the scope creep.) Even if you're working on the base C3, I recommend building in optimised for testing and then backporting to work with the original C3; that's what I did for the Equip/Relic screen (in two phases, actually, going Optimised->Original and then Original->BNW).

B-Run is right, though, that the biggest issue is space. The descriptions themselves are the big things, but then there's also (if you're using more than the existing event bits) you'll need custom eventing to set your bits. (Extra event bits aren't an issue unless you're already playing around and adding things to SRAM, and even then... I mean, I got an entire additional in-line magic list for Gogo just by moving some things around, so there's definitely space there.) There's also the work of actually writing all of the descriptions.

100% doable, though, once space considerations are taken care of.

ETA: With a bit of juggling, there's even free space for at least one, possibly two or three, extra menu items, without losing any content from the main field menu.