I think its up to you if you want to release mod tools or not, you probably cant release the full C++ for your game but I dont see why there would be issues with the editor tools especially when we have access to things like SteamWorks. I certainly havent read the legals top to bottom but it seems extremely premature to be discussing this let alone making the assumption, even if it were true that we cannot include the tools or it somehow means people need to get in on the subscription I dont see that as a bad thing. Quite commonly in the mod community people will host files, sites, just a number of different things you may or may not have to sink money into, I wouldnt assume that every modder doesnt have any money or doesnt want any money. If someone is just twiddling around in their basement there is alot of tools available if they dont want to pay and for games I can see why mod content might help keep the game fresh but I wouldnt base a business model around it, if you want a F2P or DLC model I suggest you follow those paths instead rather than relying on the community to provide you with free content. It may happen dont get me wrong but the number of people who want to be paid for modding is only increasing now they actually see their work is worth money.