For some more interactions, maybe instead of sending "waves" that generate their intial positions off-screen randomly, maybe you could draw a line or something and units spawn along that line. I like the way it looks and feels aesthetically, I think maybe it just needs some more interaction. If not line-drawing to spawn enemies, maybe placing some deployables or something, traps, turrets, etc.
mockup attached for "line drawing mechanic" hopefully it's self-explanatory.
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The line drawing mechanic looks quite interesting. I'm imagining it might end up being quite a fun element. Traps and turrets could also be a nice addition. Thanks for taking time to help us out man. And brother I gotta appreciate your enthusiasm for helping us out, you went ahead and even created a mockup. Really appreciate that my guy, it clarified the line-drawing mechanic quite nicely. Thanks again for your efforts and for helping us out :)