My two worries about sandbox games:
1. They always seem to be clunkier and less polished than other games. MMOs already have an issue with polish to begin with.
2. MMO players are some of the worst people in the gaming community. Great people exist within the MMO community, but there is a lot of outright toxic folk who make themselves the head and face of the community. MOBA players are made up of ex-WoW trolls. Now think of that sort of community in a game world where they can do anything they want. I don't want to be in that world.
So you have to start putting in mechanics to limit assholes. Then it becomes less sandboxy, but more tolerable. It's a delicate thing to balance.
I am curious to see the actual game and how they handle these problems.