Really nice top-down shooter, and I love the Sega-Genesis game style title screen!
I think a good extension to the idea would be some sort of event system like waves or swarms or bosses, like the “survivor” genre (for want of a better term).
By the way, I really like the enemy movement are a bit unpredictable but still threatening, means more of the gameplay space is usable (for my entry I was very lazy and just made enemies chase the player with some inertia and ± some random angular differences in direction, which leads to the player just needing to run away constantly)