I had spent some more time playing the game after reading your reply to thing about a good mechanic to freshen things.
Just an idea but adding a couple of playable character types and also sending those guys as enemies that you could capture could be a good good twist for example. Maybe giving the player a special skill like "capturing" that makes him vulnerable for a couple seconds so its a challenging thing to do , an after doing it It gives you a short powerup and some new mechanics so its also rewarding. You could keep an going by changing bodies as you go with a new body one after an another.
This is just an example idea but its this type of stuff that could take the game to a different level for me. It already feels to good and addicting but adding a element of relief and change to the game could change a lot.
For example yes even now it feels amazing to shoot those bastards for minutes. But the difference between first moments of the game and the 5th minute is only that there are more bullets in the screen. Plus after playing a couple of real long runs It tends to start repetitive. With a mechanic added that makes the player breath every now an than with a different mechanic in the game loop would make it feel fresh for a way longer time.