I'd definitely be hyped af for 9.0 if Blizzard went with this. I'd keep old expansions available as optional leveling paths, i.e. when you finish your race's starting zones at level let's say ~10 (post squish), you can keep levelling up in the old world or you can go 10-60 (assuming that 60 would be the new level cap) in e.g. Northrend.