I would love to see more VP chip cards (or Duration cards, though Donald has been pretty unequivocal in stating there will never be any more), but I would guess chances are slim. They'd have to put another set of VP tokens in the box, since each expansion comes with the bits that are required for that expansion - so that you're only required to own that expansion plus one of the stand-alone sets.
Let's see if I can rampantly speculate on a mechanic that might be a focus for a future expansion.... How about, goal cards? (OK, I had already guessed that wrong for Cornucopia.) Powerful, optional one-shots? (Like Mining Village.) Tokens that can be accumulated and used later for basic stuff like +Buy or +Action? Negative coinage on powerful attack cards?
If I remember correctly, Donald X (and feel free to correct me!) doesn't like to make one-shot cards, because a lot of people hate one-shot cards, apparently. There are still only three, being one each in each of the first three sets.
What I've found is that each expansion, besides adding a new mechanic, actually produces a paradigm shift in how the game is viewed. Intrigue added choice, but the addition of mixed-type cards made you think of other possible pairings. Seaside added Duration cards, but really made you think about, in a more subtle way, preparing for your next turn. Alchemy added the Potion, obviously, but cards like Scrying Pool made you really think about Actions, and the Philosopher's Stone was the first Kingdom Treasure, making us think about having additional Treasures in the supply. Prosperity added Victory tokens and more Treasures, as well as Colony/Platinum, but the paradigm shift was the expensive cards - I've got $8; do I get a Province, or a King's Court? Cornucopia promoted diversity, which really is its shift. Rather than just spamming Grand Markets, Cornucopia makes you want to have a diverse deck. Hinterlands, with the when-gain cards, builds upon Seaside's "next turn" mentality, as well as adding yet one more thing to keep in mind when purchasing cards. Noble Brigand is there - do I really want Gold/Silver? Ill-Gotten Gains is there - do I really want to skip the trasher or Watchtower?
So, what paradigm shift will the next expansion bring?
