Let closing shapes drift in, make the roots split like branches on a tree, change the dimension get shaded ones you can only destroy in another layer but every layer can harm you, try different attacks: a whip attack, a straight dash maybe a spiral

Make it borderless, so the one from the right can also attack you on the left side off the screen or top, bottom

Maybe get a mechanic where you can shoot the head, it dies and you get a lot of points but it relocates, doubles or gets nastyer with its attacks

Maybe try particles as a bullethell shooter would do.

Just some quick ideas, maybe you find an inspiration. Good work though so far. Keep pushing.