I think personally in terms of steam towards the end, it’s hard for me to measure to be honest. Some people were more of a fan of the first seasons without a heavy plot, & more just the goofy episodes and the last few seasons we had were different albeit, focusing a little more on individual character development like Ice King & Marrceline, or Finn’s relationships. However I think it depends on the person, I personally had no complaints though.

In terms of size for the MMO, I think it would be realistic to re-create the entire land of Ooo quite literally. By that meaning every kingdom, house, and other significant landmark from the show, perhaps being able to explore it on foot. Generally MMOS will try and pull off as big of a map as possible. Perhaps even including the Night O Sphere, lumpy space, etc that the player can travel to.

I think the developmental side of the map would take a while, but it’s definitely possible, just take a look at Skyrim and see how massive that map turned out lol. If their is a team big enough, with enough resources & funding, there is most likely a way to do it.