Currently, as devs, we are limited to what exactly we can do when it comes to designing (G)UIs. Despite there being a few options, it seems focus has primary been put into over ui control interfaces as opposed to customisation.
Few examples of a greater range would be-
Custom / More Font options.
‘ClipDescendants’ working on more than just a square shape, with this we’ll have a greater ability to over lay and apply masks to our GUIs.
The capability to customise the awful preset ROBLOX shapes. Currently we can insert a rounded image; but only as blue, grey, or Black and red. Limiting us greatly.
(( Although there is the plug-in Roundify))
Capability to tween // animate individual text box texts: transitions if you will.
Those are just a few ideas I’ve thought off, please apply more if you have your own.
There’s already been requests for some of the features you listed
Please support the existing requests
Roblox has several shapes in their texture folders that we can use in games, one of those is a rounded rectangle rbxasset://textures/ui/RoundedRect8px.png
However I find it very much a hassle to constantly go through the textures folder just to find something that can be used, so I support this request.
Roblox also released a survey regarding UI tools recently, but these most likely will take a while to get added.