I predict at least two more type combinations that have not been on any official card will appear.
...What are the plausible ones? (And by "appear" do you mean be selected, or just be submitted?)
Treasure–Attack seems like the one with the most room for development—and indeed, one has already been submitted.
Treasure–Looter (or Treasure–Attack–Looter) similarly seems fairly unobjectionable.
Victory–Looter is possible (x points per Ruins in your deck?), though it doesn't seem very likely to me. Action–Victory–Looter seems more plausible than just Victory–Looter.
Reaction on a card with more than one other type (Action–Attack–Reaction? Treasure–Victory–Reaction?) has nothing wrong with it in principle, though you'd have to take care not to let card effects get needlessly complex.
Treasure–Duration also seems reasonable, though it causes hairy rules interactions with Herbalist and a few other things.
Victory–Prize is a possibility if we do a new Prize, though remember Donald X playtested Victory–Prize and found it not worthwhile. ("Worth 1 VP per Prize in your deck"? Eh, it's just winner-wins-more, and probably balanced but not very interesting.) Action–Victory–Prize or Treasure–Victory–Prize could have potential.
I don't think Action–Treasure has a realistic chance of being effectively designed, though I guess anything's possible.
Anything–Curse is a non-starter ruleswise. A kingdom card saying "Setup: in games using this, replace the Curse pile in the supply with" could make a mixed-type Curse more feasible, but (a) LastFootnote's contest rules don't allow for this, and (b) this is what Looters and Ruins were invented for anyway.
We're clearly not seeing new Ruins, Shelters, or Knights.