I’d like to stress the last part of @DeReel’s post, especially as it connects to @igorhorst’s point about boardgame solo play.
From time to time, I play a lot of boardgames solo. Sometimes it’s just to learn the rules, sometimes it’s to enjoy a game I like a lot but don’t get to play often enough, and sometimes it’s because otherwise I wouldn’t get to play it at all. The latter is especially common with wargames, and solo play is a big part of the hex-and-counters wargaming hobby.
However, what is very clear to me is that solo boardgaming is much easier and much, much more satisfying when it’s easy to identify the sides in the game as their own individuals rather than as players external to the game. For example, it’s much easier to do this with a WWII war game where you may have the Axis and the Allies as the different sides compared to a card game like Dominion where there is no character or faction for the player to identify with in the game. (I believe this is what is called the “agent” in board game theory, but trying to google that veers off into other directions so I haven’t been able to verify that right now.)
So to relate this to roleplaying games, I believe solo play there may benefit from identifying clear agents the player can switch their point of view between. My experience here is much more limited, but I’ve had more success with solo play in games where there is a lot of character vs. character conflict than character vs. environment and I think the fact that the former better enables the perspective shift between different agents (i.e., the different player characters) is one factor in this.
Can this be applied in a way that makes more character vs. environment oriented games easier to play solo? Perhaps it helps if you imagine the GM as a separate character? This isn’t something I’ve tried myself, but I imagine it could be easier to split your perspective so that the imaginary GM character is the one who is the author of the character’s adversity, while you as the player of the character try to overcome that adversity.
For example, let’s say you’re playing a solo character in a space opera game. When you come to a situation where you need to make a GM type decision, I imagine it would help if you have a clear picture of your GM character as being Bob, who loves big space battles, chase scenes, and over-the-top meolodrama, or Frances, who’s really into politics, ancient precursors, and mystic visions, or Lisa, who’s always looking for a chance to bring up cool robots, intrigue, and psychic powers.
Anyway, just a thought.