I think unlike 1 where it felt a little self-contained (not totally because of obvious cliffhangers, but Star's wand being cleaved at the very end setting up a dividing line between first and second), 2 sets up stuff that seems set to also continue across 3. That doesn't bother me, yet, because I think it shows they have more of a long-term plan.>1) Why did Ludo want all the corn? Why was he grinding it up?
Mewmans have corn, monsters don't. If Ludo/Toffee are trying to start up some war in the background this could tie into it somehow. Food supply for an army, food shortage for the other side, something. They're kind of keeping us in the dark right now, but with Ludo still being shown to have the rats it seems that plotline's continuing.
The grinded corn at least seemed to be used a bit for that model of Butterfly castle the rats were practicing attacking, which seems like it might happen in the premiere.>2) Is Miss Heinous going to do anything?
I'm more confident about this than the other two points, since she's one of the card suits. If they hadn't gotten a renewal for 3 she probably would have done something more in the second half of 2, but as it is now the Heinous episode seemed mostly to remind the audience she's still out there.
We'll probably get some lore stuff related to her down the line. How she's connected to the Butterflys, her monster fetish, etc. Maybe she's Eclipsa's descendant.>3) Is Mina going to be involved in any way (she's meant to be a Mewni champion)?
Would've said no last season but since she's in the new opening she might be; at least one reappearance. Battle for Mewni might even show her as a warrior with Moon in her more lucid days.
I think all of those things will be addressed now that we have 2 more seasons; whether it will be done well is the real issue. So far I don't think there's anything set up that's been completely ignored.
Their story arcs now seem to last more than a season and so might be moving slowly.