I would welcome a return to early 2000s human interface design. Everybody trying to emulate or surpass Apple took us down the road to ruin.
Besides, I think the sentiment that if it isn't flat and minimalistic then it is old-fashioned, betrays a bizarre lack of imagination on designers' part. Any time I mention skeuomorphism, 2.5D, shadows, density etc, people say I live in the 90s. Why? Are these features so onerous that one cannot conceive of a modern, or even trend-setting design that employs them? To me, this seems ludicrous. Designers brag about innovation and creativity. OK, then, here is a challenge: stop aping Apple and design tomorrow's skeuomorphic, dense, slick, rich Desktop UI.
I just want a really solid, wholly original 90's style high-relief theme for GTK/Cinnamon and Qt. I used to use the old 2000 theme on Windows XP and 7 with some darker colors, it looked and felt great.