Some more Achievements would be nice. Kind of weird that you don't get anything from beating CH 11+. Also some Achievements between the lvl 500 to lvl 1000 and lvl 1000 and lvl 5000 Achievements would help encourage players to continue grinding for levels as well as softening the gap between rewards.
Suggestion: Create a few new enemies that appear at later rounds. Have them do something unique such as having a very far standing range or something that flies higher up on the screen. I feel this would make the game a bit more interesting later on and offer the possible need for a bit of strategy.
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You are 100% right, working on more enemies and bosses soon :) Especially super bosses mini bosses, etc. Thanks for the feedback!!