*Seamless building to Android and iOS
*Improved mobile performance
*Seamless integration with no problems using iAP, Ads, Google Play/Gamecenter
*Major improvement to plugin support
* New features (can't think of anything at the top of head)
These are a bit general but really good suggestions. I saw other users asking for similar improvements.
Construct 2 is a great basis for all this, but please don't waste time/effort designing for 3D. If I wanted 3D would use Unreal engine (which already has event driven support called Blueprints) or Unity 3D (which has well developed plugins for Events). Do not put yourself up against these behemoths and focus on making 2D game development the best.
+1
On a side note if you MUST grow past 2d games, try adding better support to creating different type of Apps (fitness, maps, social networking, etc)
It would be interesting to see Construct 3 move towards making apps as well and not just games. I'd love to see a focus for apps too. We can make certain types of apps with Construct 2 as well, and while I think it's the best way to have Construct 3 as game development focused as Construct 2, a few features designed for app development woudn't hurt if Scirra have time to implement such.