“Classic+” is really bad for Classic. The reason is simple, if you add on content that didnt exist in Vanilla, it isnt “Classic” anymore, it is a new version of the game. So, the “no changes” crowd would go away if you did that and probably do more private servers, which Blizz would not want to see happen.
Class changes were extremely good for WoW. In Vanilla, you had effectively one role for every CLASS. That made things quite boring at times. In TBC, it came down the intended roll for each SPEC. Some of the builds were not good, however, you know what you would want to do with them.
Flying was intended as one thing in PvE, a level limiter. What I mean by that is you would not get to areas not intended for people at 70 and sometimes with heroic gear. It did work, but as druids got flight at 68, I got to see how hard some of those areas were until you were 70. Were there other consequences later in BC, yes. However, you still got to see a lot of people as you leveled.
As a side note, did you ever have to get a dungeon group together in TBC?
My biggest opinion is still, Classic+ is a really bad idea since it goes against the idea of “Classic”. I am doing classic because I can do some stuff I enjoyed there. I am not playing to get raid stuff done, just to enjoy what I am doing. However, I have friends who never enjoyed Vanilla and will not play until a new TBC comes out. Then I will go from having only one other old friend being around to all 5 I still keep in contact with after we all left WoW for a while. Not everyone enjoyed Vanilla and it would be a good idea for other expansions to come out on their own servers down the line.
More servers with more types of content and gameplay will be a good idea. One server type removing “classic” would be a bad idea.
As one last item, if they added the ability to play on old Azeroth and let you turn on universal scaling on retail, I would love it. I liked the old world a lot. I like a lot of dungeons and raids you cannot do properly anymore, even the Cata heroics. If they added on the ability to reset all quests, I would do it in a heartbeat.