I love mashing games together, and do it all the time. Sometimes it’s to fix a game that isn’t working for us. When we played Full Light, Full Steam, the “scripts” just weren’t working. I think in this case, it was because we were doing skype play without a shared virtual tabletop, so it was just too hard to pass pieces of paper around, or even to see them so we knew who to pass to. So we replaced them with some bits lifted from Primetime Adventures, and the game ran much more smoothly for us.
When Amaranthine wasn’t working for us, we rebuilt it using big chunks of Shadow of Yesterday.
And I often start out by creating a mashed-up game. Currently, we’re playing “Smallville“, set in a modern-fantasy alternative world, so the characters are wizards (and possibly fae or the like), rather than supers. I’m a tinkerer, and I’ve played a lot of games, and read even more. So pretty much whenever I sit down with a new game I have to force myself to play it as written, because I almost always see something that I would’ve done differently, or that I’ve seen done better somewhere else, or just a bit of the system that I think my group would enjoy having more or less complexity. (Nowadays, I usually try the game as written before changing anything, because I want to try new playstyles. But if it’s a game I’ve played before, all bets are off.)
But probably the most extensive example of this is my Ars Fantasia rules. I wanted the feel of Dungeons & Dragons and the rules of Ars Magica. So back in the mid-’90s, I sat down to make it a reality. A few years later, I updated it for use with D&D3E, and we actually played a long-running campaign using the rules. Two, in some sense—first someone else switched our existing D&D3E game over to those rules, then a few months later, when he was tired of GMing, I took over, and all the same players and many of the characters transitioned to a Spelljammer campaign.
I can’t share these rules, because they’re full of copyrighted content, plus they’re horribly incomplete: you need to have a copy of Ars Magica and a D&D Players’ Handbook in order to make a complete game. What i wrote ended up being around 80 pages of content. Some of those rules are just reproducing content from elsewhere, in order to cut down the book-flipping a bit, but a lot of it is new stuff, translating D&D content into Ars Magica terms. On the upside, 3rd, 4th, or 5th edition Ars Magica and AD&D2 or D&D3E will work equally well. In both cases, the differences between the editions are mostly in parts that get excised in the process of mashing them together, and those parts that actually vary (such as the spell details in D&D) work equally well—you just end up with a different feel, depending on which edition you use as your basis.