I actually have a pretty extensive list of ways wow (classic in particular, i generally consider retail a lost cause) could be retrofitted into more of a sandboxy, classless, factionless game through the use of the skill system (the same ones that govern weapon skill and professions) and the reputation system, along with various ways to implement classless design through the use of selectable talent trees.
Imagine: players can swap between that grant abilities, armor proficiencies, and class utility that one might expect from said tree. Imagine being able to pick any 3 spec talent trees from any class in classic and making that into your “class”. You could access any tree you like, but only 3 at a time, and each has to be levelled up individually. Getting any 3 to max level wouldnt be all too difficult, but if you want to unlock more talent trees, you need to work on leveling them up, or maybe tie certain trees behind raids or other content (like, if you beat nax, you can unlock death knight trees).
I could go on for days honestly, but I’m on my phone in bed right now
Well, the hallmark of a good game is easy to learn, hard to master. How much that is the case I think would come down to the nuances of the design, which would be hard to really figure out without actually building it and fine tuning it, seeing what works, what needs to be changed, for example, I would have a set of “core” talent trees, and then “hero” trees that cant be combined with one another, just so you dont have n^N possible combinations that you need to balance together, each new talent tree would only need to be balanced against the core trees
Oh, and yes, you’d be able to respec. Not sure where I’d draw the line on it, definitely would be free, but I also wouldnt necessarily want people just being able to swap entire skillsets mid dungeon/raid? Again, one of the nuances youd have to test and play around with.