I was thinking about these ideas during almost all the time I've played Infinitode 2:

1. Add a button to upgrade the tower at a lower cost. When farming and trying to complete quests, you're thinking about how to place towers and use bounty modules, not upgrading towers. And it seems that more towers with lower level are greater (by dps at least) than a single fully upgraded tower. So I think it would be handy to distribute coins evenly among towers. I'm doing it manually every time. So this button will help me (and I hope - help other users too) decrease these boring updates. I think it is suitable for this game to unlock this button by some research (like by ~5K infiar).
2. Possible target for the tower "nearest by rotation". By this target - the tower will select the target that will need fewer degrees to rotate to point to. It should be very useful for a minigun. Also, other interesting targets: "farthest" (should be great for a sniper with ultimate) and "farthest by rotation" - (for cannon with "Foundation" upgrade. At least it will be fun when the tower rotates on max degrees after every target:))
3. Add an icon on sources when all 6 slots for mined items are filled. It's like for a custom map when resources were consumed, but on another side with another color (I was thinking about brown-grey, but I'm not a designer) with different icon. It should be great with a custom map when you add a lot of sources for mining and sell/buy new miners when nothing is left in the old source:)