Some feature that would be great to have would be a backend for webgames, then scirra handles leaderboards, achievments, IAP (yes IAP in webgames will be a thing!) and maybe even Advertisements!
I know that some features like leaderboards is not so difficult to add on webgames (we can use databases like mysql, etc). But a unifiqued plugin that handles that and could work with GooglePlay and GameCenter would be very straight forward. So using this plugin when exporting to web, it would use the Scirra backend, but if it is exported to mobile, it would use this respective backend (Googleplay or GameServices).
Who knows Scirra are already developing a thing like this? If not, please consider!
Just this alone would convince me to purchase a C3 subscription. (none feature announced until now, although are very interesting ones, did not convinced me yet.)