OK, here’s what I’m working with at the moment. Totally open to critique!
(The dot is you.)

It’s a set-pace side-scrolling “runner” (sorta) where you are some type of organism (undefined, may keep the whole thing abstract) traveling at a constant speed toward the right. The controls available to the player are the up and down keys, which they use to move between three different lanes.
There are two “phases” of the game: constructive, and destructive.
In the constructive phase, the player is picking up dots that will change the composition of its organism. There’s nothing that harms the player. By picking up lots of a certain color of dot, they develop greater immunity to that color. Switching to picking up different colored dots will give an increased immunity to that color dot, but result in decreased immunity to other colors. Immunity is expressed as a percentage, so you could be 20% immune to red, 20% immune to blue, and 60% immune to green, for example. If you then changed to an all-red lane for the entirety of the rest of the phase, it might result in an organism 100% immune to red, and 0% immune to blue and green.
In the “destructive” phase, every dot that the player runs over that the player does not have 100% immunity to will do a little bit of damage, (inversely?) proportional to the player’s immunity. Therefore, the player wants to strategically switch between lanes so that they’re running over mostly dots that they’ve developed an immunity to.
These two phases presumably repeat, but to what effect, I’m not sure.
I’m hoping to make the game a little more interesting by having, in lieu of music, a background drone, and then having the dots play different notes within a chord as the player picks them up (although time will tell if I even have time to attempt that).
The part that scares me most is that this is probably going to involve A Lot of Math. Not hard math, but math. And I’m not sure I’ve ever done anything even vaguely resembling procedural generation before. But that’s what LD is for, hooray!
(I would post a picture of my breakfast, since that’s apparently a thing, but I haven’t eaten it yet. Going to go do that now.)