If it hasn't been said beforehand, I'd love better folder management. In C0.x... er, Construct Classic, if you want to put an item in a folder, then you have to do it from the folders tab instead of simply moving it in the objects tab.
The second thing is probably a plugin that would only draw a portion of an image in a similar manner to Game Maker's draw_sprite_part() function (left, top, right, bottom). It'd be useful for tilesets, textures, etc.
The third one, of course, is to see to it that animation works for other plugins than just Sprite. An animated tiled background or dropshadow would, I think, be pretty cool.
...Oh. And primitive shape drawing. That would help too.