Just brainstorming, but would it be possible to pull out the MTG-specific portions and make it just an online card game framework where people can make their own cards and rules? If people choose to use it to copy MTG that's on them (see Tabletop Simulator, for example) but they might also use it to create their own new games.
The challenge is that the level of complexity with rules interactions in MtG is absolutely enormous, especially if you want to allow the freeform format, which allows all sets and cards.
Oh I didn't implement a rules engine, that would be impractical. It's a freeform platform that gives you all the tools needed to use every mechanic in the game, but it's up to you to actually follow the rules. No different from playing in person with real cards. It's sort of like a drag and drop sandbox with a lot of extra features around the edges. It's a vastly improved version of Untap.in.
If you do not implement the actual rules, then it can probably be made to support many other kind of card games too.
However, I would be interested to have a FOSS rule engine of Magic: the Gathering, preferably written in C, and licensed by AGPL3 or some compatible license. But, unfortunately there is no official FOSS implementation; if there was (at least of the rule engine; not necessarily the UI) (especially if literate programming is used for the rule engine), I think that it would be better since the rules can be made more precise.
I would want to ensure that the rule engine has no pictures at all (you can easily download pictures separately if you want them, anyways; so the rule engine doesn't need them and shouldn't have them).
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.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_.... than 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.
