Good game! I got to wave 47, and only really stopped because I couldn't die and I was reaching like 5 fps.
The contract system is fun in theory, but it felt like the best strategy was always to pick the big contract, because as long as you kept picking big contracts, you would have enough power to always survive them, making the risk-reward part of the game fall sort of flat for me. I really liked the design and mechanics of the black cat things, and would love to see more content past wave 10! I also think it would be fun to add more variety to the modifiers, besides standard risk-reward changes (for example, having a modifier-exclusive enemy could be interesting, or if that would be too difficult a modifier that changes the distribution of the enemy types). I also really like how your fire rate increases as you get close to 0 time, as it leads to more close calls, but my strategy meant that I didn't really get to engage with that system very often.