Of all the suggestions I received for the Reader's Choice Challenge I think this is one of the best. I wish I had thought of it, but it came from Trey, author of the From the Sorcerer's Skull blog. I really could take this topic in any direction! However, I think I will once again start with.... *DRUM ROLL*... flavor fiction!
Captain Scarvo, dread pirate captain of the Hideous Wench, the most dreaded pirate ship with the most dreaded pirate crew in all of the Dreaded Sea, was looking through his spyglass as the merchant vessel tried to out-maneuver the pursuing Hideous Wench.
"Arrrrrrrrrr... ye won't be getting away today!" Captain Scarvo turned to give some orders to his dreaded first mate when there was a flash of light in front of the ship.
Most of the crew gasped, and Captain Scarvo moved forward to get a better look. He could hardly believe his eye. Rubbing it, he then lifted his patch and rubbed his "going below deck" eye. "What in the name of Neptune's nephew is that?"
Redfinger, his first mate, shrugged. "Captain, it looks like a... a... giant floating silver clam shell!"
The crew, surprised to point of inaction, watched as the giant clam shell drew closer. The sun glinting off it shell blinded some of the crew who then shaded their eyes.
Pointing at the clam shell, Redfinger drew his cutlass. "Captain something..." A flash of light interrupted Redfinger, and he disappeared. A few ashes dropping to the deck of the ship where he had been standing.
Captain Scarvo screamed. "Turn this ship about! All hands on deck!"
When I was first thinking about this topic, I thought most genres can work together well, if a little thought is put into the campaign, whether it's for a short period of time, a story arc if you will, or a permanent campaign feature.
You could have horror be the primary thrust of your science fiction game. Aliens anyone? I always considered it to be a horror movie. The later ones seemed to have more of a sci-fi flair to them. There should be plenty of source material for horror in your science fiction campaign.
Horror also works well in a superhero campaign. Actually, most genres mix well with superheros. Of course I don't think it works to well in reverse. A Call of Cthulhu campaign might not work so well if all the players are superheros? That would be a bad collision I think...
Horror seems to work well with most other genres. Unless of course you're running a My Little Pony RPG for your daughters. You don't want them finding a Puzzle Box and summoning a Cenobite!
Now it's harder to mesh science fiction with fantasy. Especially if you're running a "hard" science fiction campaign. Most dread pirates don't stand a chance against an alien starcraft with it's advance technology. Perhaps you could have some fantasy characters abducted by aliens and taken to their home world.
I really don't think sci-fi works too well in the western genre. However, in the superb comic Texarcana, there were two aliens, or were those demons? Oh well... So maybe if it's limited it might work. If you want tech in your weird western, then you need steampunk. Not too much of a collision there...
When word of caution. If you're running a particular kind of campaign, and decide to, say for example, have an invasion of giant metal alien robots in your Medieval Fantasy campaign you might want to clear it with your players first. If they're expecting fantasy, don't change horses midstream, and surprise them.
Of course if the players want to "jump genres", then a high fantasy or post apocalyptic campaign would be best. A Gamma World or Mutant Futures, with the planar travel mutation makes this easy. A spell, or magical gate would work in a fantasy campaign. Then again, it would probably work well in a science fiction campaign, if there is a technology that allows travel between dimensions, but that's probably a whole 'nother blog post.
Mixing, mashing, or whatever you want to call it, campaigns can be fun, if it's what the players want, and you are prepared to run such a campaign.
Captain Scarvo watched as the ship flew into the night sky. The little grey men had requested a parlay, and Captain Scarvo meet with their leader. He had no choice. Half his men were killed in a mostly one-sided battle. All he had to do was turn over his gold to the creatures, and he and his men were free to go.
Shaking his fist at the sky, Captain Scarvo vowed one day to have a ship like theirs. Then he would have his revenge.
"Arrr... I'll be the dreaded Pirate Captain Scarvo of the skies!"