I agree with @Derp that you have to start with your basic goal. Not just a setting (though that's important too), but a vision.
I think that a TV series is a good analogy for a MUSH. Start with the elevator pitch that you would use to "sell" the game to execs - only in this case it's selling to prospective players.
Then make decisions about your cast. Not specific characters (unless it's going to be a roster game), but general roles and focus. Think about things like factions, PVP, organization.
Then make decisions about the story. Unless you're going for a full-sandbox mode, you probably want to have at least a "season" worth of ideas sketched out. Think about what the characters are going to do between events, and what systems/structures you need in place to support those things.
Then think about the OOC atmosphere. How will the players interact with each other? This is where things like consent, PVP, plot guidelines, etc. come in. Also, are you looking for a more narrative free-form feel, or a more crunchy immersive feel.
Only when you've answered all of the above should you make any decisions on server tech and code. Ares has a lot out of the box and is easy to set up, but it's not a good fit for every game. Nor is FS3. So you should pick the tech that fits your vision. It might make a lot more work for you getting the place started, but you'll save yourselves headaches in the long run.