As a desktop user…there’s honestly nothing much I’d like to see in a redesign. And I mean that in the literal sense; the site’s design (on desktop) seems pretty much fine to me, it doesn’t need a massive overhaul.
I suppose my biggest complaint currently (if you could even call it much of a complaint) is that the site UI can feel a little cluttered, so maybe just reorganizing where buttons are/better grouping similar buttons/hiding some stuff in a drop-down or expand/minimize section. I know the bottom half of the top bar (“Images”, “Activity”, “Forums”, etc.) isn’t used a whole lot at least by me, so maybe those could be hidden in a drop-down menu on the upper part of the top bar (similar to the account drop-down, or heck, even part of it for all I care), and just expand the upper portion’s size to fill in the gap left by the removal of the lower portion.
Anything else would be more-less functional additions that don’t actually impact the design. Two things in particular would be image titles & live formatting preview on comments.
One thing I would not like to see, though:
For the love of Celestia, pleast do not make the UI all rounded! (A.K.A. what the Openbooru design did on a partial level.) Every site and their mother does that styling these days, and I’m sick of it. I’ve always liked the fact that Derpibooru has avoided that trend and kept the UI very rectangular with hard corners. This includes rounded profile pictures; just keep them square, it won’t kill you, I promise.
Finally, the last suggestion I’ll make is to please leave a permanent way to revert to the old/current UI for anyone who can’t stand the new one. Or at the very least please revert/change things that get a bunch of negative outcry.
I’ll be sure to make further posts as I think of things I want to see or do not want to see. I might even try to use Inspect Element to do mock-up screenshots of what I’d want to see.