My idea for how a MMO version of iM@S would be like... Just a pipe dream, this is never going to happen in a million years.
In the iDOLM@STER Online, you don't produce 765. You don't produce Cinderella Girls, Million Live, or any of those. Instead, you create your own idol in a very in-depth character creator. You can choose from dozens of personalities and voices for your idol, and even play around with the pitch a la The Sims 3. And of course you can make more than one have a unit.
The gameplay will be much like the console games, including the raising and communication, except with a multiplayer aspect. When you go do a festival, you're actually competing against another player's idol. But you can also make friends and collaborate with other producers, such as pairing up your idols with them.
Costumes would be given out through gachas, and you can also get special idols through gacha, including 765 idols.
The song selection would be all standard songs from the console games like Go My Way, Kiramekirari, Ready!!, you know. There might be a song gacha, too.