Mechanics
I have always been in favour of fiddling with the Curses, but I understand such a thing is very hard to balance.
I've talked about Curse / X dualtype cards in the past and about a card that both gives Curses and does something with them.
The trash has been considered a no go area, but I think it could be used for some cards.
Theme
Ok, so we started with our little kingdom, used some magic with Alchemy, had some political issues with Intrigue, roamed the seas with Seaside, increased our wealth with Prosperity, celebrated our diversity with Cornucopia and ventured to the borders and beyond with Hinterlands.
What's left?
I like either a clear "light" or "dark" theme for the next expansion. For light, something like a Carnival (too bad Jester's already gone) or some other festival (also gone) with cards like "Brewery", "Square" and "Minstrel". For dark, it could be about revolt, disease and poorness with cards like "Beggar", "Pied Piper", "Plague" and "Quack".