new ideas:
1)maybe after completing the game you unlock sandbox mod or something like that where you just can have fun with all the units in the game, manually place defenders e.t.c
2)having debuffers would also be nice, monsters that can disable defenders attack, reduce damage or attack speed, make them miss attacks e.t.c
3)maybe some more information while choosing monsters? for example Nearby Death, how much is the radius exactly? btw I feel like it needs to be buffed for everyone because it's extremely hard to use it.
4)maybe some upgrades besides monsters? for example: "recharge time of first 10 deployed monster is reduced by 1" maybe it should depend on cost also, for example golem's recharge time will be reduced by 3 instead of 1, but it can't be reduced below certain amount.
5) being able to sell monsters also would be helpful.
6)about boss monsters, I had an idea that boss will spawn after certain amount of dead monsters, depending on their prices, for example: you buy a boss, it costs 25 gold and to summon it, enemies must kill units for 100 gold in total.