Use Gamescope.
Resolution scaling cannot work at an API level, pretty much any game released since like 2004 needs to be aware of the actual size of render targets and the only thing that just making them bigger will accomplish is to break literally everything.
Nothing extracted yet.
Many games don't support MSAA even when forced and don't have any scaling option. Would it be possible to implement a resolution scaling system, just how dgVoodoo2 has for example?