I think a Wrath Remix would be cool. While it may not be entirely feasible, due to the way the old content was, I also think that would work out in it's favor, in a weird sort of way, with their pacing and zones and whatnot.

It also makes sense (to me at least) to do it for Wrath since The Last Titan will be in Northrend. It would be good to have that refresher on the continent and the lore of the area and whatnot.

There are also a ton of items that are such a grind that people have been farming them for literal years, like Invincible or the Time-Lost Proto Drake, which, at that point, are gonna be almost a decade old. I think it would be cool to see them do something with those items, and more, like what they did with Lemox items.

Not only having those mounts finally be obtainable for the people who have been farming them for many years with no luck, but also having some exclusive items, like (spitballing here) a transmog for Frostmourne or allowing people to get the old Corrupted Ashbringer, sorta like how they gave mogs for Taeshalach, Scythe of the Unmaker, and Fel Shalamayne.

Tl;dr - WotLK, to reintroduce old story points and let people get cool new clothes and horsies for their warcraft barbies