From what I've seen in this forum there used to be a vivid interest on role-play groups themed on the Roman empire/republic so I thought to point you towards a writing/game group I discovered three years ago. The group's name is Legacy of Sulla. As you can fathom from the title, it's a role-play group (well, more of a political game than a role-play group, actually!) set on the twilight of the Roman Republic.
The main goal of the game is to make your player-character the first man in Rome by accumulating the most influence points. To accomplish that your character will have to climb the cursus honorum, propose/debate/vote and pass bills in the senate/assemblies, host games and races, make and break alliances with the other player-characters and, most importantly, lead the legions to war. The best feature though is that you get to do all of the above within a system: the game has an elaborate rulebook where everything is explained in detail, so you know what you can or can't do. It's very similar to a computer game, really, with the only difference that you write and get write back about what's happening instead of seeing the outcome in graphics.
I think that the best element of the group is its verisimilitude. I know this must sound weird when I am talking about an open-ended game but, aside some major historical characters like Caesar or Pompey played down so that there's space left for the players, the rest of the era is true to history (which is a great thing for history geeks such as myself!). What's more, I found the administrators of the game to know their history, so you'll get to face incredibly plausible situations. I believe that the term 'alternative history' suits legacy of Sulla the best.
Game aspects put aside, I find Legacy of Sulla to be a source of entertaining and luxurious writing so if that's your thing you're going to love it for literature value alone. The group has already a successfully concluded scenario under its belt played out from March 2008 until December 2010. The new scenario was launched in March 2011, so it has only a few months of life - but that means you get to jump in from ground floor.
I really recommend you give this group a look. Here's the link: http://www.ancientworlds.net/aw/Group/1206480
Be sure to check the rules pdf found on the front page, as well as the Senatus thread for some better idea of how the game is being played!
Thanks for reading!