if Mima ever for some weird reason gets to appear as a "reformed" former evil spirit/soul/deity/Marisa's mom, I can see her using PoDD's hyper mode against the player :p
I'll hope for unlockable characters (though then it would be a bit too much shot types -- 2 per character, at least, from the screenshots), which means more endings and -- of course -- more shot types and characters

(more unlockable stuff from Spell Practice, too). Problem is that youkai aren't usually playable characters when it comes to solving incidents (exceptions being IN and PoFV.. and maybe some assistance in SA and UFO).. not that this means that spellcard rules can't be used by youkai as playables, though.
I wonder how will ZUN make 10D become distinct from the other games (one gimmick for example being Eirin and Kaguya, and the Prismriver sisters participating in combination spellcards).