Could you say what all the themes are for all expansions? I have a fairly good idea, but I don't know if it will fit anywhere...
Intrigue• Victory and Victory hybrid cards
• Cards that give you a choice of multiple options
• Cards that interact with Victory cards
Seaside• Duration cards
• Cards that affect the top of your deck
• Cards that affect your future turns in some other fashion
Hinterlands• Cards that do something when bought or gained
• Cards that do something when you buy or gain other cards
• Cards that filter (Embassy) or that push filtering (Cache)
Dark Ages• Cards that do something when trashed
• Cards that care about the trash
• Cards that upgrade other cards
– Cards that use Ruins (Looters)
– Cards that use Spoils
Cornucopia• Cards that care about variety in your hand or deck
– Cards that help you get variety in your deck
Guilds• Cards that generate Coin tokens
• Cards with overpay
– Name a card
Alchemy• Cards that you need a Potion to buy
– Cards that care about Actions
Technically Cornucopia has cards that help you get variety, but Donald X. said that he'd like to have another card in the set to push variety, and I agree. Any Alchemy card submitted should be good in multiples.