In the GrimDark Future, There Is Only Racing
You're a racing team in an over-the-top cyberpunk/post-apocalyptic future. Some of you are drivers, some are mechanics, one or more is possibly a financer (we'll have to talk about where exactly the team you put together gets its operating capital from), and others might have more exotic skills, depending. You might run more than one character so we can be sure to have at least 1-2 drivers and a mechanic in every session.
Each session is a single race. Activities include: Repairing/upgrading your vehicles. Repairing/upgrading yourselves. Haggling over parts & medical care. Arguing over who gets to drive in the race and who's fault it was the the XL-SR5 is banged up and won't be ready for the next one. Advancing personal subplots. Investigating the other teams. Sabotaging the other teams. Dealing with sabotage by other teams. Doing shady-ass things to get cash. And, of course, driving!
System: Probably GURPS. I'm familiar with it, most of the potential players are familiar with it, it would have sufficient crunchy detail to make the vehicles interesting, and if it doesn't already have a crunchy racing system it wouldn't be too hard to come up with one.
In the GrimDark Future, There Is Only Home Invasion
I forgot to mention-- I have Leverage, the RPG! It's about running cons & heists. So yeah. You're a crew of conmen and thieves in an over-the-top cyberpunk/post-apocalyptic future. Every session is organized (in a way partially controlled by the system) around a heist. Exactly how Robin Hood vs. grimdark mercenary you are is up to you.
System: Leverage, obviously. With a few hacks to make it Cyberpunk ready but that actually should be super-easy.
In the GrimDark Present, There Is Only This Shitty Bar Full of Vampires
You're all neonate vampires. You're connected because you all hang out in the same bar-- for various political reasons it's safe for neonates there. Probably vampire owned, possibly an official Elysium. You are kinda sorta friends, in a human/drinking buddy kind of way. Vampire life is organized around feeding rights, parceled feudal-style by the Prince to the politically connected or personally useful. Your problem: You don't have any feeding rights, so you have to beg them from your sires/other older vampires in exchange for dubious favors.
Activities include: Feeding. (It won't be played out every time, but definitely the first few will be done in detail.) Bargaining for access to other vampire's demesnes. Holding up your end of those bargains. Sneaking into demesnes you're not allowed to feed in, and dealing with the consequences if you get caught. Tying up loose ends from your mortal lives. (Or ravelling them out, as the case may be.) Maneuvering to get granted your own demesne. Possibly, investigating suspicious-ass things and getting involved in vampire politics in other ways. Negotiating with each other about feeding rights/other issues.
System: Vampire: the Requiem, naturally.
Regular Fucking D&D
This one's still kinda rattling around in the back of my head so I'm not sure if it's ready to run yet. But I was reading the Forgotten Realms book the other day and I was all like "you know, I've never run a 'normal' fantasy campaign, but I bet I could do that."
Broken magic castle would be a good basis for a dungeon, yah. And I guess if I'm going to do "normal" D&D that implies that there should just be like a little village nearby and a bigger city somewhere further away.
System: S&W/OD&D (or ACKS, now that I think of it)