For a prototype is game is very solid. The stun and reload system is very good addition for this person-based shooting game style. Added the fact that you can stop shooting briefly and manually reload is solid. Frenzy feels right without being overpowered, and the skill selection is great for difficult curves and challenges. Maybe a more personal thought this is, but if Frenzy secondary was a heavy shotgun with a large, wide damaging muzzle plume and the pellets could bounce once. I think players can get more tactics out of it. If you really want to push the Frenzy SG - let it charge up >:3
Back to the game workings, the traps add a bullet hell feel - so definitely keep that. So for keeping gameplay fresh and spunky I see two things thus far. Using switches and special breakable terrain to affect horde flow and create breathers (Environment Mechanic). The second and further of that is finding cool things in the fields.
A little something like: anage to uncover and protect a Sp target in the map, gain a new way to go and its bonus points. Push a big red button, turns out to be something cool and aids when the horde gets VERY large. "Easter Egg Bonus-ups"basically that make the map/environment just as fun.