I would just like to point out this annoyance that could be addressed at some point in the future.
Popups and UI elements (as in a Pause popup, an on-screen menu, all sorts of game toolbars etc), as do all sprites, currently "leak" touch and mouse click events downwards. So if you click an "ok" button in a dialog box, it will also activate all touch sensitive game elements that happen to be beneath it. This is undesirable for, hopefully, obvious reasons.
I know this is by design but I deliberately use the word leak because, frankly, I think you need user input to stop at the first (topmost) encountered object way more frequently than you need it to work across multiple stacked things. In fact, I struggle to find a use case for the latter.
While this can be manouevred around by switching off groups and juggling variables I think it would make way more sense to have a dedicated layer for UI elements or a setting in layer properties to disable touch and click events propagation.
As an example, I have a game that has a lot of rather complicated inter-related swipe, touch and drag and drop events and some buttons and pop-up messages that appear on top of it. It has already became quite a dancing routine to keep these from interfering with each other. It also introduces clutter with all the logic designed specifically for this purpose.
I don't mean to rant (too much) but I imagine most games will have things that appear on screen that are not directly game-action related, so this might be worth considering so that C3 continues to be one of the most developer-friendly game creation tools out there.