EDIT: Rewritten for clarity.
As far as the 2 things I miss (custom Menu's and Toolbars). I don't think MC
needs custom Menu's (The Menu's are fairly well organized). It is a nice thing to have - and it's likely the easiest of the lot to implement --- as parsing what-amounts-to an INI file to generate a menu's contents is fairly straight forward --- since AFAIK most of MC's internal functions are exposed to the API. And while I do miss custom Toolbars. I think MC's bottom Button Bar could very likely fill those functional needs with much less work, as it would take advantage of the existing infrastructure.
Requirements:
- Multiple Button Bars.
- Hooks that can cause Button Bars to change, e.g. via Keyboard (Hotkeys|Modifier Keys), Mouse or directory changes|paths.
- Dialog to manage Button Bars.
There was a tool that I used to use, TwinKey, you could set folder paths (regex|glob) filters, and trigger events|changes.
E.g. One might filter on: "C:\DATA\Music\*\*\*", which would match: "C:\DATA\Music\ACDC\1980_Back in Black\" and change the tab color|tab icon or the background image of the file-panel, etc.
If we also consider:
1) Multiple Bars.
2) Multiple Root|Parent Bars.
3) with Child-Bars, that can inherit Buttons from the Root|Parent.
If you set the first row of a Root Button Bar as "Default Buttons", then it's Children BB's would automatically inherit and display those buttons. So that when you change|toggle between various Button Bars you would be able to, in effect, cause contextual changes to the MC interface.