Okay, my two cents:
When you begin, you are offered a choice: explorer, constructor, empire-builder, citizen. More could be added, like soldier. When you click one, you are given your avatar, with the name USERNAME_NUMBER. So I would become GHAO_1. If, once you've exited and reopened, you decide to be something different, you change the number. So username is always obvious, but you can have multiple avatars depending on what takes your fancy each night. Suppose I started as an empire-builder, GHAO_1. Next night I choose the same, the username GHAO_1 remains. The next night I am a citizen, I become GHAO_2. As more options come through, the end number increases, but to show-up the developers, they get a special insignia (like cameni_100!) showing they can't be attacked or something (because they're testing in-game or doing something else). Maybe modders testing can enter the sandbox, but appear in mulitplayer still, albeit with the ! at the end. Perhaps.
Once you've clicked your choice, you get taken to a splash screen, with the options of designing your avatar's appearance or just not caring. Not caring either creates a random appearance or uses your previous appearance. Designing takes you somewhere similar to Sims, perhaps, with basic face shaping, hair styling, clothes choosing.
Then you're in the game. An explorer walks, drives, flies into the wilderness, looking for resources or good places to settle. Back at base (if there is one), he can use the coordinates from the chromium browser to show off his discoveries. Out in the field, he can also call (using chat functions) people out to begin salvaging, or drilling, etc.. A constructor is in charge of roads and railways. To avoid flamewars in realtime, there could be a delay of 24 hours in which any road pattern saved by a constructor is permanent. A constructor also builds the oil wells, algae plants, etc.. An empire-builder plans and rules over these. When military interaction and diplomacy come into play, the empire-builders become more important - they choose tribes/clans etc, and who 'owns' what. They can set jobs as well - suppose wood needs cutting down from XXX and taken to YYY. A citizen does these jobs. They could be a doctor ('healing' injured people, although in-game death would have to be carefully managed), or a truckdriver, or a pilot. They would use a jobs board, perhaps similar to ones used in AirHauler (for Flight Simulator X and 9 and X-Plane) to get their jobs. Once they pick one or more than one, they disappear. Chat could be used for people to request a certain job, or to ask someone else if they could do it instead. A soldier does soldier-y things.
There would have to be some sort of skill-level. If two doctors were vying for kudos (my imaginary currency, also beloved of Iain M Banks), they could brag about how good they were. Maybe it would be an XP thing. But generic stats like Speed, Health, Strength could be improved upon in a 'gym'. Intelligence stats would enable certain jobs, say to become a pilot requires more intelligence than driving a Tatra. A doctor thus would gain more intelligence than strength, and a soldier the opposite.
