I'd like to see the old talent tress revamped in a post-Shadowlands expansion and brought in as "Paragon talents" essentially replacing leveling past 60 in future expansions and eventually needing another squish. If you're familiar with D3/ESO, the Paragon/Champion Points from those are something I see working well for WoW. If you're not familiar, it's essentially just progression past max character level that offers very small boosts per point. Every level you'd have a choice between .1% Mastery/Haste/Vers or 1% Movement Speed, Health/Mana regen etc.
With how quick Shadowlands leveling is, the current trees feel quite nice while questing 10-50. You get a point every couple hours playtime that (mostly) give big power spikes. Post Shadowlands I can see it becoming bland again.