As a Roblox developer, it is currently not possible to use the number keyboard on mobile devices. Sometimes you only want a TextBox to accept numbers, and having a numbers-only keyboard for those cases would be very useful. It would make it easier for players to type in number values, and help clarify that you are only supposed to type numbers in that TextBox.
This could be added through a new property on TextBoxes that determines whether the regular keyboard or number keyboard is used.
This is how the number keyboard looks on iOS and Android.
If Roblox is able to address this issue, it would improve my development experience because it would make typing numbers into number-only TextBoxes easier for mobile players.
Credit to @DevSpec for this idea