For the most part, I agree with TheBigA. However, you can also make an argument that the mid-late 70s was also a very fractured time in music between disco, rock and 60s holdouts.
Hot AC covers Hootie, Alanis Morrisette, Matchbox 20, Goo Goo Dolls side of the 90s.
Modern Rock still plays Pearl Jam, Nirvana, Bush, etc.
Rhythmic AC covers Mariah Carey, Boyz II Men, Backstreet Boys, etc.
A few years ago, I used to think that an all 90s pop based format could be viable, but not so much anymore. I would have thought somebody would have tried it by now, and to my knowledge nobody has. Have you noticed that many Hot AC stations, even in 2008, still have 80s weekends or lunch hours and very few have local 90s programming? I know of a few Hot ACs that were experimenting with specialty 90s programming, and some of those dropped it because it just wasn't testing as well as 80s music. As far as Hot AC goes, it seems like we heard much more 80s shows in 1998 than 90s shows in 2008.
Modern Rock is different though. There are many modern rockers that have 90s lunch hours or the occasional 90s holiday weekend special. Eventually the day is going to come, probably sometime by 2015-2020, when modern rock can no longer get away with playing so much 90s gold. About the same time, more classic rock stations will have to embrace the next generation of classic rock.
Therefore, the most likely 90s based format I see succeeding is a newer breed of "classic rock" that centers around core artists of Pearl Jam, Nirvana, Bush, Red Hot Chili Peppers, Stone Temple Pilots, The Offspring, etc., much like the "Lucy" channel on XM.