I'm sure Scirra are doing their best to make C3 the best engine it can possibly be. Even if none of the things listed below are part of C3 I'm sure it will be a great engine.
That being said, the following listed items are just features I would personally like to see in C3 or that I think would make it a better engine overall.
No. 1 - The abillity to write events in Javascript
I know part of the appeal to construct 2 is that it is a code free editor but I do feel that being able to actually write code would be more comfortable for some people (including myself) whom do actually enjoy the feel of typing away on a keyboard rather that coding purly with a mouse. Even if the code editor did little more than serve as a front end for the actual event system. This would also make it easier for users to share code with each other online. (Mabey extend this to TypeScript and CoffeeScript too)
Just to clarifi, I don't want to get rid of the current editor just adding another option.
No. 2 - Linking multiple eventsheets to a single layout
This would just be a simple way to write less but more efficient code across multipule layouts. For example; if a game had multipule layouts for levels and had custom code for how the playable character moved but each level had some unique code for certain level specific mechanics then it would be nice if you could write the character movement on one sheet and link it to multiple layouts and then give level mechanics there own eventsheets.
I know there other solutions to this problem such as writing all the code on one eventsheet and checking the layout name before running certain chunks of code but that also has it own problems such as...
No. 3 - Being able to make objects retain variables and values across multiple layouts
Say I wanted to make a character jump into a portal (to another layout) and keep the arc of the jump going on the other side. Or I wanted an object to hold a value (such as lives or score) across only apropriate layouts and drop or reset it on others.
Again I know there are other round about solutions to this but I just wanted it to be on the list.
No. 4 - More options with the defaut objects
Stuff like adding a multi-jump limit to the platformer object instead of just a double jump and adding an option to make an objects movement go in steps etc,etc. This one's pretty simple.
And lastly
No. 5 - Customising the look of Construct 3
Some people prefer a darker look to their editor and some people prefer a more classic look. Giving people the option to change the visual style of their copy of C3 can only make people happier.
This would also be good for visually impaired people if they wanted a high contrast mode.
And there you go. As I said, this is all only my personal opinions and should only be taken with a pinch of salt but if any member of Scirra see's this, just know that I think you've done a great job with C2 so far and I will raise a pint to your work on C3