I'd personally like to see PvP System / Battlegrounds, but have everything AFTER THAT, so 1.6-1.12 be slow progression.
If it's all or nothing, i would take waiting for PvP for the sake of slow progression tho.
The only thing worth about experiancing TBC again for me would be with full AQ/Naxx gear, instead of levelgear.
Rushed Progression Vanilla is actually the one thing that DESTROYS the whole reason i would want progression into TBC for.
EDIT:
Also, the most likely releasedate for "WoW Classic" is November 24th, 2019, to have it launch EXACTLY 15 years on the day after original Vanilla Release was for US servers.