Most logical sequel idea
Mockumentary.
If they were to do a Wayne's World 3 twenty plus years later, wouldn't a mockumentary work best?
Seeing an older Wayne and Garth running around on misadventures may not work, but an older Wayne and Garth being interviewed for their old hit public access show, along with little snippets here and there of Wayne and Garth up to their old activities (yet, not fully) serms like the best possible choice for a sequel. It would even help clue us in on what they've been up to for the past 20+ years.
And if it seems they still have it, perhaps a more proper sequel would follow?