Good and important question!
I think we should do several things:
a) Godot should cooperate with an engine independent proprietary asset provider. (But this is disputable. This is the old question about short-term tactics "making it popular" (open source attitude) or long-term strategy "sticking to libre content" (free software attitude))
b) We should help opengameart.org (OGA) (Money, producing assets, ad, art jams)
c) Godot could integrate OGA assets and assets from public domain sites into the engine (probably in the form of an extension).
d) If there is any missing art production tool in the free software cosmos, we should close this gap. But the only missing tool I am aware of is that Gimp currently is more complicated to use than Photoshop. But even for that exists already a project. PhotoGIMP seems to do exactly that.
I think there's even more, but I can't quite grasp it. So this is it, but possibly (a) is not good.