Coop mode for old singleplayer games

The amount of effort it'd take to do that would probably be better spent making a game that's actually built around multiplayer from the get go instead of awkwardly shoehorning it into something never meant for it. As for the industry becoming stale, you could always ask if anyone knows about obscure indie games that do something similar to what you want. That way, you can be part of helping them succeed despite their lack of marketing or quantity.

So fundamentally changing (from the code up) old games, to show how they were...before the change?

People already play what they want, the old games are as available as they are... with a PC, virtual machine and emulation you'll never run out of good games. Without twisting them in the name of "originals" or whatever that idea is o.O

coop mode for subnautica would be awesome