This thought came up in another thread, but the more I thought about it, the more curious I became about the topic.
How would you go about it?
Let's pretend that you're tasked with creating a (relatively) mechanically mainstream RPG that starts with the assumption that it will be played 1 on 1 ( one GM, one character player).
It's going to be action-adventure largely, regardless of genre. Our hero is going to be getting into trouble a lot, and a fair bit of it physical danger.
By mechanically mainstream, I just mean that the sort of general stuff you see is present: stats, skills, hit points/wound levels. A little dirty-hippy, maybe, but mostly recognizable to someone not familiar with SGs.
Once you've got that, how do you modify that if/when more players want to participate in a session?