I expect a big time lawsuit where someone has to explain to a judge that their game about rolling d20s and adding modifiers to them is totally different from WOTC’s game about rolling d20s and adding modifiers to them.

So this is connected more to board games, but I can't imagine it's legally different for ttrpgs: my understanding is that game mechanics are impossible to copyright or patent and very hard to keep competitors from ripping off. Wotc copyrighted "tapping" a card by turning it sideways to show it's been used in Magic the Gathering, but in MY card game you "exhaust/utilize/bow" a card sideways which is legally distinct. WotC can force people to work around them but they can't actually prevent people from making third party content compatible with their game.