If Nirvana never existed, where do you see the rock scene heading in 91-92? Since Alice in Chains and Soundgarden had pretty much been established around that time, while stuff like faith no more and pantera also were huge, i actually see a movement of "thinking man's metal" getting big. Since AIC and Soundgarden sounded more like metal than grunge (especially AIC), i can see their popularity spreading to similar bands who had a more intelligent message like metal church and voivod, with maybe warrior soul, kings x, and savatage representing a more commercial (or at least mainstream) part of that scene. The heavier bands who were considered alt-rock (nine inch nails, helmet, gruntruck) would also be a big part of this scene, and i can see the thrash bands still thriving as well (maybe metallica and megadeth wouldnt have gone as commercial, and maybe second-tier acts like testament and kreator could get some of that popularity too via latching on). Of course, this movement would be a slow-grower, but it is realistic.