Nice work, Expressicist.
It is illegal to make even a free version of someone else's thing without their permission, that seems obvious. I am not too interested in discussing legal stuff; it always tends towards, how much can we screw you over without getting sued?
And it definitely puts the nail in the coffin of creating an Isotropic clone.
I have a few questions about this. As of now there are at least 3 working open source Dominion simulators, one of which has all but a handful of cards implemented. These are essentially free versions of Dominion that facilitate local bot-to-bot play. Are these simulators actually illegal but tolerated because they don't compete with Goko, or are they fully legal? In case of the latter, where is the legal line? With slight modification they could facilitate local human-to-bot play (in a sense they do already because you can program the bots) and local human-to-human play. With quite a bit more effort you can add online functionality, allowing bots and/or humans to compete remotely, which in principle is little different from playing locally, except that it has now become an alternative for Goko.
As for iso clones... I have no moral objections against running an invite-only server where access is granted only to those who own all the sets on Goko (this can in principle be deduced from the gamelogs). Goko won't even discover this unless someone rats.
And then there's the option of going fully rogue, either by hosting the game anonymously from a country where a US C&D order is meaningless, or through some sort of peer-to-peer architecture where, say, the current automatch pool and leaderboard are distributed across all players, and matched players connect directly to each other, storing the logs locally and communicating the result of their match only implicitly through their changed rating. Or maybe you can just use an automatch and leaderboard server, as long as the server doesn't contain the game logic it might well be that only the software would be illegal?