The following video by Josh Strife Hayes is well sourced and researched. It is chock full of details, big and small of things that could go wrong in MMO design. I could list some of the most salient, but really I would not do it justice. If the best lessons are learned from failures, then I think this is a masterclass and I recommend everyone watch it.
I like to take your ideas and supersize them. This isn't criticism, it is flattery. I come with nothing but good will and a spirit of team-building. If you take what I write any other way, that is probably just because I wasn't very clear.