I don't think you can have a patent on game mechanics.
Games Workshop will sue you into oblivion if you use their character images and names.
But if you made an online version of Mordheim with exactly the same rules and rosters, but renamed to 'Lost City' with different characters (even with the same stats), your own original art etc. then you're 100% in the clear.
Copyrights and patents are a funny thing sometimes.
The real question is: do you think it's ethical to do that.