Not sure if this has been suggested already but I'd really love to see a mode where the hero is more active. For example, currently Io is the hero given and it can ONLY build towers - It'd be cool if Io could actually use skills to help defend as well; would make the game a bit more active between build time :-)
Could stretch it further and allow hero picks as well, maybe limited to a small pool of selected heroes.
Edit: Oops, apparently I didnt search very well, already suggested :-)