Since we're already playing Gloomhaven I think a Dungeon Fantasy campaign would be redundant, even if it has a Norse flavor. Also Christopher mentioned that he's allowed to play one weekend per month so I'm thinking of running my campaign on Discord, MUD-like with 24-hour turns. Because of that it doesn't need to be a long campaign as each adventure might take months to play through.

Dungeon Fantasy / Norðlond setting - This uses Dungeon Fantasy RPG which is pretty much full GURPS minus anything not needed for D&D style dungeoneering and adding class templates. Norðlond is a Norse setting for DFRPG from Gaming Ballistic with several supplements and adventures.

GURPS 4E Prime Directive + Star Trek Adventures - Prime Directive is built on TOS and a bit more military focused but it has GURPS 4E rules. I'd adapt STA setting and adventures to Prime Directive. It would be a Federation/Starfleet setting.

GURPS WW2 + Twilight 2K - This would be a weird variant. I'd like to do something like Twilight: 2000 (which has a several Eastern Europe adventures) but on the GURPS side not that much modern day material. There is GURPS WW2 which had a good number of source books. So I'm thinking at the end of WW2 the Allies and Russia battle each other. The Allies use a few nukes which devastate Eastern Europe but causes social upheaval bringing down the Allied governments. Once again we start with the PCs as part of a US unit stranded in Poland but it's 1946 instead of 1996.

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Update: GURPS WW2 + Twilight 2K got three votes so we'll go with that.