Just a little idea I have for some time now:

How about a button and/or Keybind that pauses the Movement of everything?
The idea is that I wanted to try out different formations on higher Stages but within seconds I have enemies attacking me which hinders me in switching crusaders in the front positions.
Also if no enemy spawns I can see the raw DPS without boosts like Gryphons Legendary Obsidian Sphinx which adds x% DPS per enemy on screen.
All in one: Just a Button which disables Enemy spawning.
It´s just a small idea, not that important but would make this already amazing game a little better in my opinion.