I'm not really liking full open source at the moment because I fear it will fragment the game, i.e. several people will copy and maintain their own separate versions of the game, causing several different/diverging versions, all downloadable at different locations, and it won't really help anyone except those individuals.
Possibly a better option is for me to leave the game on Steam as a free game, and make the source code public on my Github account, (or maybe the TM Community Github account). This would retain an 'official' version of the game, and allow anyone to collaborate (make changes) to that version of the game and hopefully reduce the problem above. I would also have to allow a number of people access to the Steamworks page so that they can upload new versions to Steam as they are developed.
Also hopefully we can continue hosting the forum.
Not a bad idea.
There should definitely remain an official version, perhaps make the first person to create a version with multiplayer the suggested version, that way everyone knows where to go to, and it can also be available on the steam page?
You never know, it might even give you some more inspiration in the future if the community grows.