Thanks yall, here comes the progress I made =>
First of all I worked on a basic enemy AI for the spaceships, imagine they would fire front engines when they are colored white
I thought a little bit about what interesting mechanics would fit in the game, and I thought a slow-motion during which you can't absorb matter could make for some nice gameplay.
The slowmo isnt there for nothing, I was thinking it could bring the game to a sort of puzzle feeling, see :
I also started coding for planets, which are gonna be kind of "bosses" with a repulsion field that you will have to find a way to damage before destroying the damn place, exactly like this : 
well in fact it will absolutely not look like this but he.. can't show much more than 3mb at a time you curious little thing. Sorry for the dissapointing ending you end up with a mass of 2 planets ofc.
Don't hesitate if you have more recommandation, thanks human people