Obviously not legal advice for anyone, but the general consensus seems to be that in many countries (including the United States where WotC are based) game mech

Obviously not legal advice for anyone, but the general consensus seems to be that in many countries (including the United States where WotC are based) game mechanics are not copyrightable. This interpretation has been stated publicly by the US Copyright Office: https://web.archive.org/web/20161122174519/http://www.copyright.gov/fls/fl108.pdf https://web.archive.org/web/20161122174519/http://www.copyri... Obviously the name can be trademarked, and text (e.g. the names / descriptions of things, the specific expression of the rules) and images (e.g. card art) can be copyrighted. There is a long history of clones of games with the exact same mechanics, but with everything renamed and the artwork changed. Occasionally, there are attempts to patent mechanics. So it wouldn't necessarily be a problem if GP was to rename the game, change all the card artwork, make sure no instructions from the original game are used in the new game, and rename all the cards and concepts in an isomorphic way (i.e. a 1:1 mapping between MtG and whatever the new game is called). That way, it could be a completely new game which happens to have the same mechanics as MtG, but isn't a derivative of it legally. It would be possible to speculate what WoTC would do in that case, but hard to know for sure. Most likely, they'd probably just ignore it if it isn't using their trademark, artwork, descriptions and so on. They could send a blusterous C&D letter to try their luck if they were particularly worried. If the GP got such a letter and ignored it, or replied explaining that their game does not reuse any copyrightable elements, they'd probably just back down. If they really decided to take it to court, they could try to hope it goes more like https://en.wikipedia.org/wiki/Spry_Fox%2C_LLC_v._Lolapps%2C_Inc https://en.wikipedia.org/wiki/Spry_Fox%2C_LLC_v._Lolapps%2C_.... than https://en.wikipedia.org/wiki/Data_East_USA,_Inc._v._Epyx,_Inc https://en.wikipedia.org/wiki/Data_East_USA,_Inc._v._Epyx,_I.... (although they'd probably have a harder road given Spry Fox was largely based on the visual look-and-feel, which would be a longer bow to draw when saying a computer game infringes on a card game). Obviously, none of this is legal advice, and the circumstances could differ depending on facts like where it is hosted / where GP lives, and the exact details of the game engine or how similar artwork was.