Well, you pose an interesting question here. Yes, I believe that you would have some form of copyright in your game, since copyright is automatically granted to creative expression which is fixed in any tangible medium. So, your expression would be covered by copyright when you typed it into the computer, published it online, and whatever else. On the other hand, I don't know how much protection game mechanics get necessarily, since copyright doesn't protect "any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work." I suppose there might be certain features of your game mechanics which might qualify as expression, but that's really a question that would need to be analyzed by someone with a good knowledge of what your game mechanics actually are.
An interesting corollary to this first question is whether or not you would yourselves have been infringing upon the rights of Capcom (and possibly others) when you created your card game, since it seems unlikely that you got a license from a major game manufacturer to create a derivative work using their trademarked and copyrighted characters. So, even if you happened to be successful in suing your alleged infringer, your lawsuit could simply attract the attention of Capcom, who could possibly sue you for infringement. Again, this is a question which would need a good deal of analysis from a professional who is aware of the exact extent of your use of those intellectual properties in order to answer the question of whether or not you would have much exposure from any infringement there.
But the basic answer is that, to the extent that this alleged infringer has copied any expression which could be protected by copyright, you could have cause for legal action. I suppose that's not a terribly firm answer, but not knowing or assuming any additional facts, it would probably be misleading to give a more definite answer.