Apprentice Wizards
Basic Premise: The PCs are apprentices in the service of a cranky older wizard. Mostly their apprenticeship is doing chores for him, picking up laundry, going grocery shopping, etc, while he occasionally uses them as living mana batteries and teaches them magic tricks. The exchange is service for spells.
To start with the PCs won't know any magic. Learning it is, in general, a slow process of dribs and drabs; the campaign is really focused on being sent off by the master to do adventures for rare reagents and obscure artifacts so Old Man Wizard doesn't have to go outside or talk to anyone.
This'll be GURPS, by the way, but no experience with the system is necessary.
There are four possible options here, sound off with which one sounds the most fun:
1. Modern world, interdimensional adventures. You're all 21st century humans who have discovered someone who knows REAL FRICKIN' MAGIC. Usually the crazy old hermit just has you pick up his dry cleaning and go to the post office, but sometimes he has you jump from world to world to get those regeants and artifacts. In addition to the multidimensional adventures you're having, you'll have to decide whether you want to try keeping your mundane life or giving that up and becoming an old weirdo like your instructor.
2. Fantasy world, interdimensional adventures. Like the above but you're from Aber-Toril or Oerth or Yrth or something. You've got plenty of medieval life skills but are probably scared of vacuum cleaners and personal hygeine (that's how the devil gets ya!) But hey, at least you know how to use a sword even if your conception of a shower is "put on more perfume." Yes I know this only really applies to medieval Christiandom but it's funny, no I won't make a big deal of how gross and dirty your characters are, this probably won't be a factor.
3. Modern World, Modern Adventures. Don't like interdimensional adventures? Screw 'em! Instead you'll learn magic while dealing with organized crime, rogue psychic, lovecraftian cultists, uh... maybe the Joker, I don't know. But it'll all be in sunny modern (your city here, unless London, in which case not quite so sunny).
4. Fantasy World, Fantasy Adventures. Like the above, but with more dragons and fewer cell phones.
Seriously help me narrow this down guys. Thanks!