The problem with '90s nostalgia is that the '90s never really went away for the most part, unlike the '80s or '70s. The earlier decades were stomped flat by '92 and '82 respectively, but nobody in the '00s came along and said, "Ugh, the '90s were stupid". Yeah, we might have said "Dude, M.C. Hammer and Vanilla Ice were so dopey" -- but we said it in 1993! With that said, the Iceman and Hammer can still be fun. I even went to a Vanilla Ice live show in 2004.
My money for '90s retro is on stuff that actually did get forgotten. I'm bearish on Green Day, Seinfeld, Nirvana, Jay and Silent Bob movies, Bush (the band), flannel or Tupac -- still too popular.
Instead, I'm bullish on stuff like Eazy E, Carmen Sandiego, Twin Peaks, Gravity Kills, the Mortal Kombat soundtrack (really any '90s dance music that makes you think of a multi-level, sweaty club in an old warehouse with flaming barrels), 3DO, Skidz Bibs, Eerie Indiana, the movie Double Team, Hypercolor shirts, mini trucks, teal/purple Oakley pullover jackets and doom metal.