I don't think it's a dumb idea at all. Though, I do have a suggestion for you.
You're going to have to make a choice, and while neither is a perfect choice, only one will be right for your game:
PVP or PVE
Are you going to allow the players to faction-up and decide to war against themselves, or are you going to provide the players with a target to rally against?
When going into putting together a theme, pay close mind to the trends of MUers and how they approach PVP/PVE.
PVP allows players to faction as they choose and tends to create pockets of roleplay. Also, due to the risk of players getting upset, factions on a PVP game tend to avoid each other until absolutely necessary. People tend to get upset when one faction becomes popular, and when this happens, they tend to throw their weight around. A good example of successful PVP factioning are the old Star Wars games centered around Empire vs Rebellion.
PVE can have player-led factions, but will require the game-runners to provide the players with information that keeps the setting relevant. If players have nothing to do, they'll usually end up sticking to relationship roleplay and playing house, which will leave you with a semi-decent population of roleplayers, but your proactive, theme-building, DOING SHIT roleplayers might taper off. So, if your focus is going to be PVE, you have to be prepared to do 2 very important things:
- Provide very good information as to how this PVE works. WHO are the bad guys. WHY are they being found. HOW are the PCs threatened by them (and after doing this, be prepared for people trying to solve the problem in one scene. Be strong)
- You're gonna have to be proactive. Tend to your garden. If the PVE bad guys are something you throw at the PCs when there's nothing else going on, interest will be lost. So, be prepared to have a loose outline for stuff that bad guys might do, and keep it loose. If the PCs fail, you have to be able to dance with their failures and not railroad a story.
So, a setting is great, your setting works just fine, but when it comes to original themes, your main task is finding a way to help others invest in the setting through either available information or immersive RP. Then, make sure you have a game plan as to how the PCs are going to congregate.
PCs are like farm animals. You need to give them drama and tasks to perform or they're just going to end up taking up barn space and fucking a lot.