Some more sophisticated dialogue and choice management, and dialogue branching (like, say, Adventure Game Studio). And a layer system. But honestly, you've come so far so fast, this seems like nit-picking. I was going to ask about animation and sprite sheet support, but you're already doing it! Bravo!
EDIT:
It is still a bit volatile at present, but you might think about Inochi2D for the future. It is an open source alternative for Live2D, a pseudo-3D puppet app for games (and, presumably, visual novels):
It does make characters seem more alive, if used well...