this thread about tasty-lookin' win2k/macOS9/beOS-era GUIs mentions that today's high DPI displays mean we can't simply return to that era, where every icon was a delicious little simcity building and every pixel was a big load-bearing statement. it reminded me how much the current feeling of being "stuck" in the current bland iOS-google-electron is due to, well, capitalist realism but also the ineffectiveness of computing's corporate masters at driving new visual and design languages. and folks, i think we're gonna have to figure it out on our own, by making a huge range of weird experimental probably-ugly stuff and seeing what shakes out.
in the late 90s and early 00s i was actually a big flat UI guy, way way before it was cool or trendy. i thought the beveling on win95/macOS8-era GUIs looked kinda tacky and busy and unnecessary. but unlike the flat hegemony we ended up with decades later, i envisioned takes on "flat" that were all about clarity: clearly identifiable interaction areas, elements you could read out of the corner of your eye where that was useful, and actually reducing visual clutter instead of just drowning everything in huge margins of whitespace (which obviously wouldn't have flown back when 1024x768 was the going desktop resolution).

the past ~15 years of GUI history are undeniable, though: touchscreens basically did away with the concept of clearly defined interactive areas - even on desktop where they're still important! the flat trend is essentially about absorbing the damage wrought by touchscreens, and murdering any last hope of discoverability. the mississippi rivers of whitespace surrounding every element are partly a way of sweeping all the problems of mis-clicks and ambiguity under the rug. "buy a bigger screen phone, jackass!" we practically hear apple and google yelling at us.
what i'd love to see more of (and maybe there already is and i just don't know about it! please link in the comments if so!) is bold stabs at desktop UI visuals that aren't mostly about trying to also meet the needs of touch/mobile (remember when GNOME 3.0 made a bunch of weird decisions for a tablet UI they never actually ended up making?)
the retro GUI thing i particularly think might be worth revisiting is gradientsgradients. there was something a bit show-offy (look, we can do 256 whole colors!!) and naive about a lot of 90s GUIs' use of gradients, but the cases where they managed some restraint honestly hold up quite well IMO:

haikuOS here isn't a "retro GUI" obviously, but it's hewing very close to the visuals of its ancestor, beOS. i really like these soft grays! the yellow only-as-big-as-the-window-title titlebars pop out nicely. the beveling on the buttons is way more understated than the Chicago or Copland languages.


as a rookie game dev in the early 00s, lightwave always just looked cool to me, i liked its style. the gradients are, again, soft; the use of different earthy colors for sections of buttons make them stand out from the 100% neutral grays, and there's a general feeling that, despite the complexity of the functionality offered, its power-to-(visual-)weight ratio is pretty favorable.

blender has already moved on from this version of their recent-ish GUI revamp. but i really like the gradients here, the sense of dimensionality they lend to each button really helps in another busy 3D tool UI. i'd try lightening up the dark outlines around the buttons. oh, and see how elegantly the rounded corners on the grouping of the Translate + Rotate + Scale buttons make them feel connected? that's the kind of stuff you can't do when you're allergic to showing button borders of any kind.

lastly, we have all those folks out there doing custom themes for the NIX window toolkits. back in the mid 00s when i first switched to linux, i spent a fair bit of time shopping for different "skins" on
gnome-look.org, which is still around! it feels a bit like the UI design equivalent of doom modding: sure, most of the efforts are amateurish, but even they have a decent density of
interesting ideas and there are diamonds in the rough that feel almost as good as anything a professional might do.
i have no idea where desktop GUI design is going, if anywhere, but i suspect it's probably not a high priority for the big capitalist behemoths given that desktop operating systems are basically worthless on their own now. so what are they gonna look like in 10-20 years? what if they looked fucked up and/or cool? let's make them regret not caring. lol