Proper font rendering is a lot more complicated than just putting glyph images next to each other…
The font rendering in most computer games, including ROBLOX, is awful (I don’t know why). I’d be more interested in real font rendering (with the libraries provided by the operating system?) and Unicode support. Of course both would require a lot of work, if they are at all possible.
In any case, allowing users to upload custom fonts has no purpose. Almost all the fonts would be copyrighted and uploaded illegally. It’d be better if ROBLOX GUIs just had real font rendering and could use OpenType fonts, in which case ROBLOX could provide a larger number of fonts released under an adequate license.