I think a graphics tileset would drastically raise the profile of the DoomRL project and consequently, yourself and your other projects. Otherwise, you're limiting yourself to your current fanbase (the relatively few members of this forum). IMO, that should be your first priority - getting out a graphical version to expand your reach to new players who may otherwise overlook the title. I can't think of any other new feature that would get new fans on board. From there you can monetize the game however you please.
And you've had me ready to help for like... 3 or so years now? You're wasting a free resource, Kornel.
This is, of course, a biased opinion, but I also believe it's a truthful one. ;)