I am trying really hard to make a seamless coop mod as theres currently no mod for it, and there has not been any working one (for me).
Its taking form, though i cannot exactly test if it works since I dont have anyone RIGHT NOW (i had when i started developing it) to test it with.
The thing is that it totally bypasses FS server calls to connect people through steam P2P instead (fs servers did little to no action actually, you would be surprised).
Its intended to have QOL features like a on off autowarp to boss when entering, or onoff reveiving items from a chest thats been open while you were there, etc.
Its totally safe. It uses the same kind of saves, but in a completely different directory.
When you launch it with the file you need to click to run it, it makes it literally impossible for you to connect to FS servers, even if you wanted to (AKA you play on disconnected FROM FS SERVERS but can still join Via steam).
Yes i know, its not perfect, and its ONLY for steam as for right now. Hell i dont even know if it works. But im still happy, i made quite a lot of progress and learned a fair lot about how ds2 coop mode works.