I believe it needs only two things:
First, dual spec. Having the opportunity of switching specs freely would add many, many more healers and tanks to the game, since having to level with them is a pain. Not to mention, it can basically double your playtime in one character, by making the gearing process much more longer.
Second, summoning stones like in SoM. Having to wait for people to walk to the dungeon all the time is just an annoyance that does nothing for the game. Yes, Warlocks lose a little bit of their charm, but they were still present in SoM and nobody complained.
That's about it. The rest of Classic is perfection.