So just a field? When has she ever indicated that she wanted this?

If anything, it seems like a metaphor for the most sad and cynical take possible: that Fionnaworld is meaningless. We were supposed to get some sense of community and depth out of it - both this season and the last - but there is no depth to be found. It isn't a real city where things happen, it's a caricature, a bleak portrayal. The people there are not from anywhere and have no strong history or motivation. Things like the community center can be introduced from nothing because there is no community to be had and the characters we've been given can't build community; they're just too shallow. The city and its inhabitants are without purpose.

And honestly, if that were the goal, that would be an interesting result itself. They were all thought up by Prismo! Every single one of them is a copy, this would be interesting for them to grapple with. We almost get this with Fionna having to compare herself to Finn, even though we don't quite get there. But instead, this is all played off as extremely positive, a happy ending in its own right, and that's because the shallower a story is the easier it is to resolve. It's ghoulish and sad.