How big is the dev team?

inti-creates already made a 2.5D system with MN9, so I'd imagine that a lot of the hard work was done there. With that said, you're dealing with content and asset generation considering the base system is set in stone.

So, in as minimal of a sense as I can imagine:
1) code monkey - works on scripts, generates values for items
2) level designer - takes assets and creates worlds
3) enemy designer - takes models and creates monsters and encounters
4) several artists for level, enemy, boss, and items.

That's probably the very minimum you can hope for with a game of this scope. So, minimalist, 7 people.