A couple of improvements could be :
-More health drops ( i've spent a crazy ammount of time at 1 hp and no matter how many enemies i kill they either drop weapon energy or nothing).
-Make bosses waekness do more damage ( i always end up only using the buster because the special weapon damage just sucks).
-There's some weird imput lag that sometimes just eats away your jumps or dash jumps.
-Some enemies have so much hp ( it kinda kills the pace of the game in some levels)