Oh, so you didn't like Magic and it's creature's general vulnerability. So that made me think of another game, that is very minion centric. Vampire The Masquerade the card game. It pretty much has no cards that directly do something to a 'creature', each creature has to perform the action, which can be blocked. Combat is also entirely card based. Actions are cards. If a vampire doesn't play a card, or piece of equipment attached, it does all of 1 damage. Now, where that comes in handy, is that for you and expansions, you could have new attacks and new blocks and counters, and they could all be usable by existing creatures. A previous set have an attack that is too cheap? You can counter it in the next set by adding a couple of counter cards, or easy blocks.
Jihad Yes, early inception of the concept borrowed from this. I actually have never played the game, but I've read the rules a number of times when researching concepts and discussed with a few players as well. Thanks for bringing this up.
(Also, you mentioned percentages, but you don't need to go that route. Fire Vulnerability X could be done as Takes an additional X Damage from fire attacks, or Fire Resistance 1, takes 1 less damage from fire attacks.)
Interesting. What I don't like about 'takes an additionnal X damage', is you could have a fire source that deals 1 damage, but a vulnerability that adds 2 or 3, and it feels a bit odd that with so little fire damage through, you'd get burned so much. I mean, if 3 damage gets through, then I can totally understand 2-3 extra damage. It feels a bit weird, otherwise, it is a good idea though.