Support 
A number of these things you have suggested, and the ones I have, have already been requested, though.
My additions:
Image/Shape Masking – Let us show/hide specific parts of GUI elements using images/predefined shapes.
Rotation Support for ClipsDescendants – Currently any rotated element will not be clipped, this is obviously not desirable behavior.
Vector Support – Because I hate low quality images.
Basic Shapes – Triangles, circles, etc, should not require us to upload images.
Well, doesn’t hurt to remind them
Also, I really like your ideas! These would be awesome.
This may sound far fetched, but maybe html UI rendering?
Yeah masking is the most important one imo
This is heavily needed. Been wanting these for a while now.
I definitely support this as a UI designer.
These would all be great. Font is especially needed right now with localisation as just uploading everything as decals isn’t really an option anymore.
DEFINITELY. I should include this in my original post. Not having custom fonts reduces the consistency in the design because we are restricted to Roblox’s fonts, while our static images have our desired custom fonts. It makes the game look more cheap and unpolished.
I’d LOVE Roblox to incorporate UI templates and samples, as well as some Google material design features, like colours, buttons, sliders, etc.
Support.
These additions would make UI designing and creation so much more efficient, quick, and better overall. I hate uploading images to get a circle, triangle, and other basic shapes, and image masking is also a big feature that makes good UI that is missing from the current UI tools.
That’s actually a pretty cool idea! Creating sliders is possible but takes a considerate amount of work to make them work fluidly.
This is on our roadmap, currently planned for late this year.
This is not currently on the roadmap. There are a number of issues here and we can’t un-ship a feature once we ship it.
We’re thinking about this, but it’s not currently on the roadmap. Drop shadows and multi-pixel text outlines are both possible, but will involve more memory usage of the font atlas, which increases the risk of running out of space in it and causing glyphs to flicker.
Currently on the roadmap.
We’ll either implement masking (above) or allow increments of 90 degrees, in the future.
Not currently planned, because SVG is an absolutely insane file format. It’s possible we could support some subset that image editors export, but there’s still issues like memory usage, rasterization time, etc. It’s all the same problems we have with text rendering, but with even less control over it engine side.
No.
We have some stuff in progress which should help with this. There’s also a great deal of cool community work, such as Roact Material ( GitHub - AmaranthineCodices/roact-material: Material design in Roblox w/ Roact! ).
Custom fonts not being on the roadmap is extremely disappointing to hear. Does Roblox plan on greatly expanding the current font library at the very least? Choices are extremely limited right now and don’t mix together at all. Every game theme will only end up using a few of the fonts.
Any new fonts we add will have to be downloaded on-demand because of space usage concerns on mobile devices. This is something I’d like to do though. Not only to improve the selection, but to improve language support.
Right now, only one font we have supports CJK languages (Noto Sans CJK), so no matter which font you pick, Chinese/Japanese/Korean will always render in that one font. We only have one font that supports Arabic as well (Arial), and only two that support Cyrillic (Source Sans, Arial). We have no fonts that support Devanagari script (as used in Hindi), etc.
Most or all of the current fonts support Spanish and other languages that only use very basic accented Latin letters. However, several fonts have incomplete Latin coverage, and will render like ransom note text in languages like Vietnamese.
There are future improvements for the typesetter planned. Many of them are waiting on the new typesetter to ship for good. Some of them are already in production, but require the new typesetter to be enabled.
If we can get cleaner and thinner fonts, such as Source Sans Light, mixed in with the same kind of lower-case G that Arial has, that would be awesome. Also having support for having a dedicated bold/italic/underline button would be a great addition as well, instead of having a seperate font for bold and italics.
We really need progress bars as an asset that can be maneuvered with a value 
think of an int contrained value but it controls the amount of progress on a progress bar.
We also really, really need dedicated rounded frames and buttons.
Hopefully the progress bars also include things like start / middle / end textures like the scroll bar.
Any update on this? We are almost half way through the next year and still no border-radius 
would love to hear the current plans!
2023 and still no dropshadows 