I think part of the problem with legacy servers is that even old-school WoW players (like myself) want too many different things from them, and can't necessarily even agree on which patch would make the best one. For myself, I tend to prefer TBC because it had a lot of improvements over vanilla, without adding a lot of the garbage from later expansions. But if I'm really honest, what I really want is a combination of vanilla, TBC, and WotLK.
In a nutshell, IMO:
Vanilla - best pacing. Yeah, people complained about having to walk everywhere til 40, etc. etc. But the game world was designed around that. Things like that which slow you down can actually be good for players, because it stretches out the same content without adding grind. It takes longer to do the content, so the content lasts longer without getting more repetitive. But the class design in vanilla was horrible. Beyond horrible, actually.
TBC - best end-game progression. Not much else to say to elaborate on it. I honestly don't think any other MMO or WoW expansion has even come close in this regard. But TBC also introduced flying mounts, which ruined the game in many ways, yet it still also had a lot of the irritating limitations from vanilla that really didn't benefit anyone, like debuff limitations. Certain things like buffs being limited to groups instead of raids, I always liked. It made it more interesting how you grouped up players.
WotLK - best class design. I felt like there were too many talent points at level 80, but the class design improvements over TBC were spot-on. I also thought dual-spec was a great addition to the game, and the changes to debuff limitations was good. But most of the rest of the expansion just made the game worse overall.
And now, if I'm really dreaming big and going all out, what I'd like to see is a level 70 cap with difficulty tuned to TBC levels, but have (as much as possible) the zones, dungeons, and raids added in from later expansions, so literally all the content in the game would be viable and relevant. Along with that, I'd like to see progression be strictly-story based and not gear-based, so the gear would all have to be side-grades or alternatives, either for stat variations or purely for cosmetics.
Now, that's my take on it, and reflects my personal preferences. But that's where the problem arises: I doubt most (possibly not even many) players will agree with all of them. So if you're Blizzard, how do you decide exactly which patch to use for a legacy server? How do you decide which QoL features to include from later patches? Most people on the TBC private server I used to play on agreed that dual-spec would have been a great addition. Would vanilla players feel the same way? How many would want it? And on and on. I want WoW legacy servers, but I'm not so sure I want vanilla legacy servers, because I can't play the classes and specs I want in vanilla, as they're not viable.