I still think Valve should take the advice of Uncle Dane.
Just hand over the TF2 Project to a company WILLING to work on it. They did the same for CS:GO, and look at how successful that game is.
3rd party companies are more than willing to work on a game in which they've given up. Especially how TF2 is still one of its top income products as of today for micro transactions.
If there were to happen, we'd get a community of creators who actually communicate in their development of adding updates and additions.
It would be a significantly good idea.