I think these are pretty obvious so they're probably already on somebody's to-do list or even being worked upon, but:
- Respect the prototype's technology_to_unlock setting - currently this does nothing
- Relatedly, "not yet enabled" functionality should be separated from "not yet able to be added to the shortcut bar"
- Allow buttons to be enabled/disabled via script, and possibly add a start_disabled setting/flag to the prototype so the button can be enabled by script instead of by a technology
- Allow toggleable to work with lua buttons, and provide API functions to read (and write) button state, presumably by having the event return a LuaShortcut entity or something
- Minor feature: ability to create custom shortcut styles (eg style = "green")
