What Systems Need More Adventure Content?

[dbm]

GURPS would benefit from more third-party adventures, however if you are seeking to publish for money (and it sounds like you are…) then SJG have a conservative approach to third parties. You could approach Gaming Ballistic who have a an existing relationship with SJG as the easiest route for that.

Alternatively, Savage Worlds has a pretty active third party publishing scene (SWAG). You could publish a fantasy adventure for Savage Worlds quite easily. They have just released Savage Pathfinder, but I don’t think that specific IP can be used yet. Might be worth considering?

 

[Enevhar Aldarion]

Just do what a lot of other companies, both big and small, have done, and do a version for 5E and a version for another system.

 

[schneeland]

Free League has an OGL if I remember correctly. So if you wanted, you could probably publish it for Forbidden Lands.
However if you are most familiar with D20 systems, I think the best approach would be to put out a 5e and an OSR version.