For example, if I and another person (working professionally) were to start working on a mod for Fallout 4 right now
How are you going to work on a mod for a game that doesn't exist yet? I mean sure, there's some subset of work you can do, but a lot of real production work would be difficult with the shipped game and tools -- there's a risk that you'll build content that can't be used, or at least not directly. There's no guarantee that mod tools for a game on a likely-similar engine won't be updated with new rules/features/data formats. In fact it's almost a certainty. Seems like a risky proposition for me, especially since (in general) you don't even know if or when the game will support paid mods.
Would Valve allow this
Why is it their problem? It's the game developer who allows mods (paid or free) or not. Valve doesn't factor into that decision.
This seems to have the potential for a talented group to actually get a piece of the release-week gross earnings pie.
Also, can a game development studio actually compete with modding groups making their own DLC? I'm skeptical...