There are 2 parts to it: incentivizing developers to make their games "moddable" and incentivizing players to become modders. So I'll do one sentence each.
Developers: Having a clear choice of sharing the game source code and/or modding tools on the game submission page, and some feature to receive proper attribution (I like the simplicity of something like Creative Commons, take a look at that experience).
Players/Modders: Having a clear way of knowing whether the developer of a game is open to their game being modded and a streamlined way of making, publishing and attributing mods.
Note that when I say mods, again, it doesn't need to be the steam workshop experience, it can be as " simple" as forking the source code and making a modification.