Every year I’ve written a garage sale  article for The Stew in which I toss out all the campaign ideas I’ve had during the year that I’m unlikely to get to in the near future. It seems that many of The Stew’s readers have a similar overabundance of ideas, since every year the garage sale article gets comments from readers detailing their own campaigns that aren’t likely to see the light of day.
Recently however, I was hit by a bit of inspiration while I was brainstorming for my “Northern Reaches” campaign  and realized that the area I was trying to find a place for was actually from my completely different “Great Below” campaign . I realized that the two campaigns featured very similar themes (exploration), technology level and setting (standard fantasy, though one was above ground and one below) so it was a natural error to confuse the two, but really: Why NOT merge them into the same game?
Looking at my list of backburnered campaign ideas, I can see further matches. There’s no reason to keep my two zombie campaigns separate, my gang warfare campaign meshes well with my demonic drug pusher campaign, and there’s no real reason that The Northern Reaches, situated above The Great Below couldn’t be on the lost worldship Warden.
The primary attraction of the campaign mashup is that instead of checking a single campaign off your “to do” list, you get to check off two or more, but there are other benefits as well. Mashing two campaigns together should provide a richer more detailed setting with more for your players to do and more options and styles of play supported. In addition, even with two similar ideas, the synergy created can push your game from good to great.
Step by Step:
The first thing to do for a campaign setting mashup is look over your list of backburnered concepts for potential matches. You can merge settings that are as dissimilar as you like. The only major deal breaker is theme and feel. Placing high tech ruins in a fantasy setting or Chthonian horrors in your supers game is fair game, but merging a high-power “kicking ass and taking names” with a grim and gritty “survival horror” game will only work if you’re willing to radically change the thrust of one or the other or limit one theme to a specific area or storyline. Otherwise, the feel of your game will be muddled at best.
I have two games I’d like to merge. One is an exploration based overland game, the other an exploration based underdark game, both are more or less standard fantasy, so they match up very well. As I mentioned before, I could easily match these two up with my exploration based post-apocalyptic sci-fi game, but I’d rather keep that one separate for now.
Once you’ve picked a few campaigns to smush together into an unholy mess, take a look for major mechanical or setting cannon problems. These conflicts could be anything that simply can not co-exist, at least not without some brilliant intervention on the part of a clever GM.
My campaigns each have several different custom sub-systems that I have to choose to discard or keep including a custom race, class and magic system.
Once those big picture problems are dealt with, it’s time to take a look at the newly merged elements of your mashed up campaign and ask yourself how they interact with the new stuff. This doesn’t have to be done with for every single combination of every element of both component campaigns, but it’s where your “synergy” comes from, so you can only overdo it by pushing it so far you burn yourself out on the game or relegate it back to back burner status.
The main bad guy alpha predators of one of my component settings are vampires, and in the other there’s a gigantic blind luminous god-fish in a country-sized underground sea. What if a vampire migrated to the underdark and discovered this god-fish, attaching himself tick-like to the beast’s back? Over the years, he has converted hapless adventurers and undersea dwellers, creating a flea-like infestation that hides in the luminous kelp on the great beasts back. Master vampires cling like massive bloated ticks and their spawn feed on their body in turn. When the fish surfaces, the plague of vampires strips any nearby towns or ships like unholy locusts before returning to their nest.
By the same token, you’re likely to run into elements that will no longer work the way they used to but aren’t the big picture problems you’ve already dealt with. These might need a bit of attention but they should be fairly minor and easy to fix.
The ghouls in my underdark campaign were previously the primary source of lumber in the underdark, which they got via raiding graveyards, feasting on the dead and repurposing coffins. Since my overland exploration campaign has no major centers of civilization there aren’t any graveyards to raid. I could just drop this aspect, but I like it, so I’ll just modify it a bit. Now the ghouls inhabit a tunnel complex under a forest, and they pour out at night with an expendable skeleton workforce to harvest trees and drag them below before the forest guardians arrive in a sort of lumberjack smash and grab.
You might need to rearrange maps and other fiddly bits to accommodate the new material, but after that and any other misc. cleanup, your new campaign setting is ready to run!
But what if…
… I want to combine crazy mismatched campaigns?
Go for it! Who am I to say “No.” if you want to run a zombie survival western set on a dying earth after the last worldship leaves focused on the romantic attachments of the PCs? Everything above is a guideline, and if you know better, not only do I encourage you to do it, I want to hear about it!
… I think this is awesome, but I only have one campaign?
Then you’re just being a tease to those of us who can never get to the end of our to do lists. Go away. Seriously though, borrow one of ours , get one off the internet, pirate one you’ve played in or run before, or pick your friends’ brains for ideas.
… there are system problems?
System is one of the things you need to consider when you decide if two settings are a match or not, but worse comes to worse, you can adapt one campaign to the other’s system, or adapt both to a more generic system.
… the PC’s are off the wall?
Since you’re combining two settings, there may be a wider range of valid PC archetypes creating a potential mess. If you’re worried about it, try a group chargen session .
… some other thing that I’m bringing up to be difficult.
Heck if I know. Ask in the comments, someone will have an answer for you.