The biggest thing I can think of that’s an “engine” thing is perhaps dyeing armors. But even then, they could add it probably without being too large an issue technically. It’s the artistic debt that’s a problem there. Anything done would require most likely reworking every piece of equipment which…is a lot. I’m sure people can say “well only allow NEW stuff to be dyed” and that’s a solution, but I think we’re all aware that will feel bad and instantly lead to complaints.
WoW2 also has a lot of problems. While I think there’s a lot to be said for a hard reset of things and to level the playing field you will inevitably get the people who want to transfer their stuff from 1 to 2. Which kind of renders the whole fresh start without baggage moot.
Then you’ll get the people who will stay in 1 vs 2 and split the playerbase. Let’s just say it didn’t work out for EQ how they expected a sequel to go. Or Maplestory.
At the end of the day, it’s a better idea to keep extending WoW until you eventually turn off the lights.