I've always been a huge fan of the BTD and I was just thinking "wouldn't it be cool if this game has a story line explaining why exactly the bloons hate the moneys or why the moneys have darts or how a super money or even sun god came to be?" just an idea :)
if there you are working on a sequel, it should have a few different modes like: survival, campaign, and i would like to see multi-player. and a few more upgrades like being able to heal your own base and turrets on your base would be nice :)