Our way of planning stuff to do is probably close to what you would guess.
We spend some time after each SDK release planning what we want to go into the next one.
We use a variety of inputs into this, including votes from the feature request board at https://expo.canny.io/feature-requests and also just stuff we want ourselves or that is new. And from doing proactive interviews with users of Expo. We especially try to listen to people who are doing really cool stuff with Expo or building really big apps with it. Jason and I and sometimes other people have also been spending time talking to people who don't use Expo to find out what they would need to make Expo make sense for them.
From there, Ben who is the team's PM and Jesse who is the team's engineering manager lead a process to take all this stuff we want to do, assess how hard or complicated it all is, and then synthesize a coherent vision for the next SDK that we then work towards.
Sometimes random things happen also, like Nikki going on vacation to India and coming back having built EXGL on a lark.
If you want to get stuff onto the Expo roadmap, post or vote on here https://expo.canny.io/feature-requests or if you have a longer rationale for something that doesn't fit there, post in the forums at https://forums.expo.io/