One of my players is leaving, or has left, and won't be back for a couple of months, if at all. It's not the first time that it's happened, and it probably won't be the last. But every time a long-term player leaves, it always stings a little. 
The last time one of our players left for a while, I resolved to try out 13th Age, a system with a lot of bells and whistles, and a lot of borrowing from different editions of good old Dungeons and Dragons. A lot of things seemed to work for me, and a lot of things didn't.  It was a decent system and I think that if we had started at a higher level than it may have been more entertaining- but asking players to analyze a game they've never played and generate a somewhat effective higher level character (where the stakes are higher) is a difficult task. It's not like any of us had played this damn system before! I think half of them hadn't even heard of it before I mentioned it.
This time, though, we've decided to do something completely different. The 13th Age game was meant to bridge time in between one of our oldest players temporarily leaving and his return while still playing in more-or-less the same setting. He took long enough coming back  that now another player has to leave, and I'm still not in the mood to run FantasyCraft again so we've decided to do something completely different.
We've elected to play Fate Accelerated, for a while, in a science fiction setting. One of my players mentioned Cowboy Bebop and Firefly, two shows I've never watched. Another player was thinking Dead Space and Starship Troopers. I'm thinking Trigun and Dune . I'm sure all of these notes will work together in some fashion.
Since we didn't have a setting created beforehand, we set out to brainstorm one. On one hand, it was a fun experience. Brainstorming with people is one of my favorite things to do, especially when they've got that spark. You know what I'm talking about- it's that electricity that happens when you all come together to make a towering pile of setting that none of you could have possibly done on your own. It's when somebody's idea gives you an idea, and then your idea gives them an idea, and then both of your ideas give everybody ideas, until everybody's sizzling with ideas.
That didn't entirely happen here, though. And it's probably partially my fault.
Part of the reason that I'm usually the GM is that I've got lots of ideas for stuff.  One of my other players is very creative and has no problems whatsoever with creating new ideas or thinking of new settings or getting things done.
My problem was with my other two players, each of whom had their own problems. I'm not getting into that here (I'm not trying to air my laundry, know what I mean), but getting ideas from them was difficult and almost painful. I was trying to stay in the background, encouraging them to speak up and give ideas, but it was rough. One of my players is very mild-mannered and the other is very outgoing and enthusiastic but completely scattered. Bouncing ideas around would go from actual suggestion to joke to digression and then the digression would continue (from mostly one person) and it was frustrating. In the three or four hours we discussed, we got done what should have taken around one hour, and then we could have "polished the turd," so to speak and iterated further on what we've got. 
Still, we got some good stuff out of it. We've got one character who's a smuggler turned legit pilot, whose Trouble is that her old smuggling buddies want her back in the game. We've got a scrap technician whose trouble is that he is socially inept. And then we have a blank space, to be filled later.
Our heroes are trying to save the colony of Orvstream, on Meskhent 3. They colony is serviced by large, cumbersome vessels, the only vessels that are capable of FTL travel. If they send a distress signal back to their homeworld, it could take years for a cargo ship to make it- the signal has to travel, then the ship must be readied, then the ship must arrive. By that time, it could be too late. The heroes have access to a single interplanetary cruiser and must figure out how to ensure the survival of the colony they all call home before it's too late.
There's some other background stuff- we all decided that space zombies (like Dead Space, but probably less goddamn deadly) are plausible, that cybernetic limbs are neat, and that prisoners with barcode tattoos and explosive collars are pretty cool. Also that sometimes prisoners who are deemed reformed are allowed passage to a colony where they are allowed freedom- sort of a Space Australia. I also have a neat image that I got from using a Traveller generator online: The image link is here and the system generator is here, if you need it or anything like it.
So I'm excited to break out of fantasy for a while, and I think that as my players get more used to sharing the burden of creation and creativity that it's going to get a little better. Each of my players has something to learn about playing games in this manner, and I think I've got quite a bit of learning to do too, which excites me. I'm hoping this works out, because I am so tired of fantasy hack and slash. Soooooo tiiiiiiired...
 Part of the big problem of playing a long-term campaign with no real ending is that peoples' lives keep going on, and you can't expect them to be there forever, you know? Especially since my current group skews as young as it does (for whatever reason that it does).
 I'll write about that later- the gist is that the system takes the 3e approach of "level after X battles," which means that for the players to progress with any speed they have to get lots of fighting done, so the game becomes "talk for a while, walk for a while, then kill things. talk for a while, fight for a while, then kill things." The rhythm is ok if that's what you're into, but after running fantasy mankillin' games for as long as I have, I could use a break.
 He's actually gone for more-or-less good, although he did come back to tell me that so it's all good. What can you do? It was nice having him at our table and if he ever comes back I'll do my best to accommodate him.
 The original Frank Herbert Dune, thank you very much. His children rifled through his notes and created cash-grab nonsense that isn't worthy of the name.
 The other part is that I get bored only playing one character, in the traditional player / DM dichotomy. I prefer a systems-light and slightly adversarial DM, where there are challenges for me (the player) to overcome using my character as a proxy. The other other part is that I really just enjoy DMing because I get to be the one who drives the game, in the traditional player / DM setup, and I like to make things for other people to play around in. I always try to give them a wide leash and enough rope to hang themselves, and in return they always manage to find a way to enjoy themselves. Sometimes I give them too much freedom, in which case I expect them to find their own solutions to their inter-party squabbling.
 I'm a big fan of iterative design, where you produce a series of drafts quickly. Basically, you set a goal, try to achieve it, and then see what you made. Look at your original goal again- is that what you want still? Probably not, so adjust your goal, edit what you already have to fit the new goal, then see what you made again. Do this five to ten times and you should have something you're reasonably comfortable with. The thing about this, though, is that you have to be able to actually make a finished draft first. If you spend all your time worrying about whether the thing you've half finished is good enough then you're just wasting time. Hammer it out and edit it after!