20 cards is enough IMO.
What I would like to see in terms of adjustments to decks might be something along the following lines:
* The ability to change decks in-game. Once only, as part of the upgrade to Imperial, or maybe as an Imperial tech. Imperial is supposed to be a huge advantage, after all, but right now, all it does is give you the ability to spend a huge amount of resources to buy some hugely expensive techs.
* Some kind of wildcard feature -- basically a card that lets you play any other card in your deck twice, or maybe even pick any card from your total selection. Each civ might have a variable amount of such cards, available by and tied to age (i.e. an age 2 wildcard can play other age 2 cards again), and costing a certain amount of resources to play.
* A random deck feature. Previous Age games featured random civilization selection. Due to the nature of AoE3, that's not possible here. However, I don't see any problems with programming a feature to make up a random deck from among the cards available to a player. It would require more quick thinking and less build-order-orientedness.
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