For now I only plan allowing to mod content, add new stuff, modify existing, not game types or creating completely different mechanics. Although reading recently, how some developers work with limitations (Earthworm Jim's flying washing machines due to lack of scripting, etc) I still could get surprised.
As for local multiplayer - I was thinking about it, as in general, it seems to be easy thing to add (at least for prototyping). But with impossible spaces it gets trickier. I was thinking more about being out of the action, seeing map, hacking things, releasing enemies etc. I don't know why I haven't thought about direct control of other characters (sidekicks! and imagine that I play Kirby StarAllies with my kids). For sure it won't be top down as it is in other games and what allows for multiple players to compete or help. If it would be added, it more likely will be FPP.
It's not on the main list of things to do but I do consider it as something that may happen later. For sure I'd like to explore this in future.
Same goes for normal multiplayer. Both coop and pvp could work pretty well.