To be fair i don't think it will be working out that well since every card has it's unique mechanic behind it and making a card where you actually can choose whatever you like can change the ful dynamic of a game.
There is a site to post these sort of suggestions by the way:
https://warlight.uservoice.com/forums/77051-warlight-featuresjust make sure to post it there and fizzer might actually look into it when it gets enough votes