[Extrarius]

I'd enjoy just getting to play GURPS, but one campaign I spent some time thinking about and tried to run once (lasted a session - the one and only time I GMed, so I had no experience and, apparently, no talent):

Initial Apparent Setting: Modern Real World
Initially, the characters wake up in a room lined with beds similar to an old hospital. They find out they've all "died" (each in different ways according to the character, each not due to any fault of the character) but were saved by an angel because they fit a needed role / meet strange criterion / etc. The characters get a long briefing, which reveals that the apparent setting is actually closer to (loosely based on) In Nomine. After learning "what is really going on", the players start doing (apparently innocuous/inane) missions for the angel. "Go get box Z123 from location B-4 in that warehouse across town and bring it back here."

Each mission, the characters face more opposition. At first, they'd face simple security systems and other technology, then unwary people, then security guards, military guards, minor supernatural creatures, and finally demons. As the characters learn more about the aspects of the world they previously knew nothing about through play, the apparent setting shifts from pseudo-In Nomine to something a bit more like Supernatural (TV Series). They're going to be facing all kinds of supernatural threats in many different forms. At least a few challenges will require fierce combat, and a few more will be 'a simple massacre, you just need an unobtanium knife blessed by clerics from 3 different dead religions'. During this phase, characters will be able to pick up supernatural abilities as powers (with "magic" based on GURPS Sorcery). I'd hope to set it up so that all characters would learn some supernatural powers to complement and supplement their existing abilities - I don't really want 'wizard' to be a primary role for anybody.

Eventually, the characters come across some kind of creature they can't find any information on at all because there simply isn't any (here) - it's from Homeline and accidentally ended up on this world somehow. It isn't hurting people directly, but it is causing a lot of trouble (perhaps tearing up buildings looking for it's mate/family/etc, and so far they've all been empty buildings because it is nocturnal and living in an area full of office parks) and needs to be dealt with. Unfortunately, nothing in the current world can hurt it much (or for long). After some work, the players should be able to find a way to travel to Homeline and either find a weapon that can kill the creature, or find a way to send/drag it back home (or even just to any other world).

Now we're at the intersection of Supernatural (TV Series) and Infinite Worlds, and there are a lot of directions it could go. One emphasis I'd make is that a large portion of the worlds available have magic at least to the level the characters know it, but far fewer have modern technology. Most worlds are advanced to the same TL overall (or 1-2 levels behind), but many would be using different kinds of magic to fill the gaps (and thus be perhaps TL ?+1 or TL ?+2).

I'd like to think the endgame would involve Cthulhu (the actual creature, not the threat of it awakening), but I've never played in a game where characters got anywhere remotely that powerful, so I'm not sure how that would work.