I got a… difficult situation on my hands.
I currently have three groups – a classic Vampire: The Masquerade game, a Pathfinder game, using a Eberron-like setting, and a Warcraft 2nd Ed. /D&D 3.5 game.
All of the players know each other and all of us are good friends. We are ten people in total, and, except by me (the GM of those three games), no player plays in more than one game.
One of the Pathfinder players will be going away, to study in another country. He used to play characters in the other two games a time ago.
Since the ten of us are good friends, and our pal is going away, it was put on my hands the task of creating a special game, with everyone, to say goodbye to him.
The original idea is to group everyone (from those different games) by means of planar magic, and drop all of them in a single dungeon. There, they must act together to destroy the big bad evil guy. Ok, cliché, but hey, it's a goodbye game!
While gluing together D&D and Pathfinder is not exactly an issue, putting together Vampires from WoD (which have completely different mechanics) is a bit of a challenge.
My original plan was to translate those V:tM character sheets to Pathfinder Character Sheets, but I have no clue where to start. I would gladly accept other suggestions.
I need to be able to conduct a few battles with everyone fighting side by side, so any answer should take that into account.
This will be the during our vacations, next month. We are going to a house on the beach, and we will use the nights to play. So, I expect at least seven game sessions.
Anyone had similar experiences on how to make this work?
Some feedback:
I picked up some of those suggestions and ran a brief simulation with a friend. @lisardggY and @Jessa worked alright, but Brian's solution gave our brief game a gigantic amount of laughs. Yeah, it is a hell of a overload of work to put everything together to work that way, but is is worth it! For short, special games like this one, it can give wonderful results.
Best Answer
Thinking of this as a one-off interdimensional travel lead me to think about a (remarkably equivalent) conceit being the center of Harry Potter and the Natural 20. In short, it tells the hilariously funny fiction of a genre-aware 3.5 wizard dropping into Hogwarts and steadfastly maintains the truth behind each universe. The imported little Munchkin, Milo, indeed works to 3.5 mechanics in the "real world."
For your thing, therefore, don't try to "merge" them. Layer the systems. Each character comes with their own system and imparts those effects on the world. Preferably as literally as possible. Treat readings of abilities as "true" rather than as mechanical abstractions designed to represent a world. Of course, use the exact same rules for the enemies. You'll want to think through the abstractions from a system specific point of view: "What does 3.5 damage mean in context of WoD, treating things literally" and so on and so forth. While this would be tedious to pull off in any kind of sustained campaign, the incongruous juxtaposition of systems could be a hilarious and fun farewell to arms.