Lets face it. Whether you are in support of changes or not. blizzard is most likely going to make some, even if they are extremely minor. Changes are bound to happen as it has been hinted at by them since the announcement.
anyway.
Changes I expect to/think will happen.
-Talent adjustment- not adding or removing talents, just tweeking their numbers to make different specs more viable when it comes to PvE or PvP. I know this is something some people do not want but it still something I expect blizzard to do, as they would want to encourage people to play this longer, and giving them more spec options to play with is a decent way to do that.
-bug fixes- this is pretty obvious, I doubt they will let bugs that ran rampant through all of or most of vanilla back in the day to stay.
-group finder- like the mythic+ group finder system, not a que system. but something that helps ease people into finding groups rather than spamming world chat (I know this was a big part of vanilla but its still something I imagine blizzard will implement)
-more viable tier gear- I expect blizzard will do with tier gear for classic what they have for tier in legion. 1 set that now changes its bonuses based on what spec you are in.
Changes I want to see
-looting through corpses I already looted- not implementing AoE (although i'd definitely take it) but I at least would want them to allow me to loot corpses stacked ontop of each other, finding that foot poking out and finally looting a corpse sometimes became tedious.
-(on the fence with this one) allowing meeting stone to be used as a summoning tool like in BC, except this time it requires 4 people to use- Again, I know travelling to the dungeon was an experience I still want classic servers to keep. but with this you can at least save a few minutes incase grabbing that 5th (usually a tank or healer) takes a bit longer than you wanted.
Changes I dont want to see
- getting loot - I don't really want blizzard to change HOW we get loot.
- professions - I loved professions in vanilla and think they should stay untouched. the whole super rare recipes that only dropped off bosses and had a 1% chance to do so made certain markets very interesting!
- LEVELING - keep leveling the same damnit!!! When I rolled a warrior back in 2007 I was very sad every time I accidentally pulled more than 1 mob while questing because I Just couldnt handle it without using a CD in that terrible green gear. But this taught me how to pull things and made me explore my class more. it forced me to be good at a warrior by learning how to use it in different and unique ways.
there are more but these are some of the big ones for me.