Some serious speculations from me as fan card maker
(although this doesn't say much, most of Donald's cards have been soo much better than cards I could think of, every expansion had big surprises to me):
Considering that the set has 400 cards, it is quite obvious that there will be "extra" (i.e. non-kingdom) cards. Donald could make another set of just kingdom cards like Hinterlands, but Hinterlands was only 300, and at the given level of complexity, new mechanics are needed. Donald always filled out the card space efficiently, my guess is there will be about 30 kingdom cards (~330 total), so there'd be space for ~70 extra cards.
What could be done with these?
- Other basic cards, as we know from Alchemy and Prosperity (although I can't imagine what to do here).
- Ancillary non-supply piles, as we know from Cornucopia and Dark Ages.
- Alternative starting cards like Shelters, maybe to replace Coppers, or Cantrips that are just added to the starting deck.
- Non-gainable cards that are used for bookkeeping (to mark supply piles - maybe too similar to Embargo/ Trade Route; to mark any player), to count stuff (e.g. how often you shuffle, how often you gained a card), or as Events (like neutral attacks that take place each round or when a player triggers it).
Guesses on the kingdom cards themselves are much harder.
The big themes that are already covered by the existent expansions are flexibility, future, currency/ Actions, buys/ Treasures, variety, gains/ Victory, trash/ cheap, and... again flexibiliy/ future I guess?
There could be overlaps in themes, but Donald always gives us something new as well.
New general themes that come into my mind at the moment are:
- Consistency (it's already covered by Prince, but there could still be things to do here.)
- New sort of interaction (Donald is very cautious with such things but maybe something a la "7 Wonders: Babel"?)
- Cards with different deck speeds (see my idea "queue pile")
- Timing flexibility (like coin tokens, just more in general, see LFns "Activation cards")
- Twist on endgame (cards that do things before you score; or special conditions for VP cards).