Cool, I was looking for a recent thread like this, I've been using C2 a lot recently and wondered if there was somewhere I could suggest stuff. Maybe a bit long to read, but here goes
Persistent Objects - Reset Game
The persistence behaviour is potentially so useful, but if the player returns to the main menu and presses 'New Game' then everything is still persistent. Only a browser refresh will fix it, which is not very good really.
New Idea - Colliders
I often use hidden sprites pinned to the player to check for various collisions, I know 'collision at offset' can help avoid this, but I've found that it won't work when you want to have gaps inside sprites or odd shapes. Also you end up with loads more objects.
My idea for this is to add something into the collision mask section of the image editor, so it works like image points. So you can add additional collision shapes to the sprite and then check for collisions in the same way as extra image points, something like sprite>overlapping backdrop at collision mask(1).
If you could have a number of these per sprite and rename them as per image points, then that would offer a lot of flexibility. No idea the technical challenges this may present to implement though.
Group & Layer Naming
It would be nice if, when renaming groups or layers, all references to them were also renamed (as per variables etc). This is not that big a deal if you're aware of it though I guess.
Steam file associations
Would like to be able to double-click a Capx without steam trying to install the free version of C2. Would also be nice if turning off the Beta updates worked without me having to re-install the free version.
Tilemap & Sprite seams
Everyone knows about this, with tile maps I know the recommended settings should fix it, I'm Ok with everything but letterbox integer scale, as in my tests it often results in an unplayable game size on smaller resolutions. The seams between sprite appear on export by design, and I guess there must be a way around this? This is probably a naive request, but on a basic level it would be nice if everything at least looked the same in the editor and preview as it does when exporting (just on the same machine/browser I mean obviously)
Music & Audio
This is the only thing about C2 that I genuinely hate. To be fair it could be partially browser issues, but this is my experience of audio:
So I can't play more than 1 music file at once, in fact I can't even change between music files; once I've started playing one then all others are ignored. Seems impossible to get music to sync with anything too, if you want that to be a core element of gameplay then forget it basically. Everything about using music is terrible, only reliable way is to put everything in the sounds folder. Of course putting things in the sounds folder means it doesn't stream. I already have to preload every single sound effect up front or they just won't play, with large music files the it's not viable on a web game. Waiting until all sounds have finished preloading until you start the game doesn't seem to work either. There also seems to be odd delays on sound files that again remove the possibility of creating something with tight focused audio that syncs to the gameplay.
Edit: came across as a bit angry when talking about the audio stuff there; it does annoy me, but for the record overall I'm really pleased with C2.