i wrote a multi-part blog post on these things i called authority and expressiveness that i think is a bit long and unwieldy now (so like don't read it, but it's linked, i know).
i was inspired to revisit this thought pattern by this tweet that @idlemurmurs made looking for systems not so focused on optimization.
in general the idea is that you look at all the options the player has available to them (in, like, what i think of as an abstract mathematics way: you don't literally look at every option, you just look at the "set of all options"), and figure out which ones appear to be most correct to the player. it gets complicated here because it's not just the system talking -- it's also the player, with different assumptions and values from other players.
if the player has one valid choice, that game is not expressive, it doesn't feel free. of course we can get into players who actively do things that feel like invalid choices (and actually it's very interesting because then if you reward things that feel like 'invalid' choices it really screws up and changes what feels like a valid choice but that's a whole other topic), but i really want to keep things simple for now.
in starseed pilgrim the player is left in the dark about so many things. this is one way of achieving a large number of equally-valid choices: give the player no knowledge upon which to make a decision.
equal rewards, or balance
you can give players roughly equal rewards no matter what they choose. so in an mmorpg, you make all the classes equally good in combat. in an RTS, you make all the races/civilizations/whatevers equally capable. in a fighting game, you make all the characters equally good.
of course people will still look for optimization, but in general this idea of 'balance' is pervasive in competitive games because (i think? in part?) they are looking to fight for the player's freedom to choose what they like. if you didn't want this, you'd make a game without those choices in the first place.
you can have a sandbox-like environment where you can do anything you want and 'no rewards' are given. this usually doesn't last too long. i've called this getting rid of 'extrinsic rewards' in the past, and eschewing them in a fun system means the player is driven by 'intrinsic rewards'. tapping into a player's own particular tastes is cool, and i think ultimately the goal of providing multiple equally-valid choices is to get players doing that: choosing their own path, expressing their likes and dislikes.
i've used a roman numeral to number this post because i might make more. this is a topic i keep coming back to.