This is the first time I've tried a solo RPG campaign. I have a setup in mind, based on the kind of backstory that I always end up making when I come up with a new character. The setup ("you are a mid-level functionary, your mentor in the palace bureaucracy is murdered and you are blamed") is not super original. But it gives a nice scope of experience for the character to drive behavior and action.  

Game System

I'm using Worlds Without Number (or WWN - note that a free version is available on DriveThruRPG), which is a nice blend of old-school D&D stats with a "10,000 years in the future" setting, where magic works because it is the remnants of technology the outsiders brought, before they were deposed a thousand years ago, and so on. The more I dig in, the more I'm liking this setting.   One aspect of WWN that I like is the presumption of competence - if a character with your background and experience could reasonably do something, just assume they did it. You should only be rolling dice for things that matter.  

Solo RPG Oracle

For guiding the story along and generating surprises for the character (and myself) I'm using the Mythic Game Master Emulator, 2nd edition, also available at DriveThruRPG. (The second edition just dropped here in January '23.)   Mythic uses a d100 roll to help answer yes/no questions, based on whether you think the outcome is likely, unlikely, very unlikely, even chances, etc. The answer table gets adjusted by the Chaos rating, which is something to track from scene to scene. The higher the Chaos rating, the more likely a "Yes" answer is, so you should always frame your questions in a way that a Yes leads to more interesting outcomes.