I think it's best to do what Tom wound up doing and have a mix. Offer identities to players who want one from the mod, let them pick if they want their own. No reason to stress on it. If you have a really good setup, by all means list the roles in advance.
Variants I've been thinking of:
CHESS MAFIA
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It's like Chess... with Mafia! There's a chessboard, town plays white, scum plays black. Every day, town can discuss and vote on either a chess move or a lynch. If town votes for a move, scum make their move back at night, no kills, no night actions. If town votes to lynch, scum kill as per normal Mafia. Both sides can win either via normal Mafia means or by winning the chess game.
Essentially what happens is you get communal chess going. The catch? Scum are participating in discussion and trying to influence the town to choose bad moves. Town should play chess nonstop until they win (hooray!), are on the brink of losing and have to play normal Mafia from there (interesting?), or someone really thinks they've found a scum (?!).
Awesome points:
Chess is fun, novelty is interesting. Can work with a very small number of players, indeed even 3-man is viable. Moves at a good clip (24 hours/day or move, or so?) and yet doesn't take as much concentration as normal Mafia, usually. Also usually not as confrontational?
Not so awesome points:
More about chess theory than Mafia theory. Can be hard to follow and unbalanced if some people know a lot about chess and others are left in the dark (though this can be avoided if people are nice). Tends to take a LONG time to finish on forums, even moving at what would be a fast pace for normal Mafia.
LIGHTS OUT
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As normal Mafia, with a little twist. Town isn't limited to just one lynch a day. They can lynch as many people as they want in succession, and day doesn't end until a majority of the living vote for ##Lights Out.
The catch? You don't get cardflips for any of the lynches until lights out occurs, and the scum have a night role advantage that's greater than usual.
Awesome points:
Novelty, of course. Wild and crazy. Tends to make for a faster game than the standard 1 lynch day. More potential for awesome swings to happen, since town can go on a lynch spree and potentially bag the whole scum team at once.
Not so awesome points:
Wild and crazy accusations may not be everyone's idea of a fun time. Can be played conservatively and just become a standard game with town at a disadvantage. Tends to work best with higher numbers of players and a large, very carefully thought out role setup, which can be hard to get.
HIGHLANDER / THUNDERDOME
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This setup runs very differently from normal Mafia. There are scum and town, as usual, but the gameplay doesn't proceed like you're used to. It is nightless. Each day, there's a short discussion phase, which must end by one player challenging another player to a one on one "duel." ONLY those two players can be voted on that day. One will live, the other will die. Winners may or may not receive role bonuses, and roles similar to those you'd find in a normal game can function either during the discussion phase or the challenge phase. Town's goal is to challenge and slay all the scum, scum's goal is to outnumber town or to take X number of heads by day Y.
Awesome points:
Lots of novelty. Game is running all the time, no boring night phases where you can't post. The head to head challenges can be great fun, especially with roleplaying, and there's potential for more crazy roles than a normal game as well. Also can work with a small number of players - like chess, it's viable here to have only one scum (although 3 man wouldn't work as well).
Not so awesome points:
Head to head challenges are not for the faint of heart. System can be hard to understand and needs an experienced mod to run it.
