My understanding is that there are currently 3 UI systems integrated in to Cocos; Menu/Items, CCControl (extensions/GUI), and CocosGUI (cocos/ui). I would like to see them better integrated, or a clear sign that 2 of the 3 are being deprecated, so that we as developers trying to help with bettering Cocos can focus on just one, which I would guess would be CocosGUI.
As a C++ programmer coming from iOS, my familiarity/preference has been for CCControl, but I could learn to use CocosGUI if that is the course of action. My issue with CocosGUI is that there is an added layer of abstraction through ProtectedNode and Widget and Layout that can get confusing, and as a programmer I prefer to add the buttons/sliders/scrollviews directly to a scene's base Layer or sub Layer. As a side note, I think there is redundancy in other places too, like I think that Node, Layer, LayerColor, etc. should all be joined into one class, a Layer with a background layer that is always on the bottom of the child stack.
As others have done, I too have created a system using JSON to do automatic layout, but that was before CocoStudio, which (as a Mac user) is still not "ready for primetime" so it's not a viable option for me yet. That being said, I would rather see the output from CocoStudio better documented, so we can tweak it directly, than see an HTML/CSS UI system added on top of the C++ version of Cocos. That said, I think Cocos does need a web view class similar to the experimental video player, so we could have an in-app browser for things like privacy policies that live on a website.
I'm very happy that CocoStudio has skeletal armature animation tool, and I am desperately waiting for the Mac version to include it. Spine becomes very expensive once you cross a certain threshold, plus requires some hackery to support 3+ resolutions (at least last I looked). As for other "for pay" libraries, the main reasons I switch to Cocos from Corona for cross platform development is no licensing fee and the open source nature of the product, and having to start paying for closed source libs would become a burden.