We need an HD version and remodel with multi-color versions of the old sets from previous expansions like TBC/Wotlk/Cata .. T5 remodels PVE and PVP versions would be awesome specially for locks.
Also T3 HD models would be insane the Wotlk Naxx ones and the old Vanilla one.
The Merciless PVP sets are great for HD versions.
Please, the current xmogs are kinda too old.
Id say mogging a full set is boring, its good for ppl just getting into the game. I only have full sets for the potential i see in the individual pieces to use across other mogs.
Def the classic mogs need to be updated, but then we should ask why the Diablo team is allowed updated HD versions of WoW Classic weapons and mogs before we do. I was highly pissed off when Diablo players not only got Frostmourne but the entire Arthas LK mog but we get a stream of kobold and fox onsies 
HD tier 3, HD tier 5 and HD grand marshal/high warlord sets would go incredibly hard.
They now make sure to only add the cool ones to the shop. I wouldn’t even bother with xmog now anyways. It doesnt even work half the time. Sometimes it does, but most times na…it does not. My pally used to change mog multiple times a day. Now I no longer can do that.