The monitor buss is very specifically about feeding the speakers in front of the person operating Ardour so multiple monitor busses don’t make much sense. What would be useful is an ability to select what you are hearing through the monitor buss. By default that is the master buss, or, AFL/PFL’d things. If you could temporarily replace the master buss with any other definable sources (other busses, I/O etc) that would become more flexible and useful.
As for multiple Master busses, as Paul mentioned, you can have as many busses as you like and freely route them where you like. You could feed the Master Buss into a number of other busses and use those as level controls to different zones. Ardours busses are really a generic thing, they could be “Aux Masters”, “Audio Subgroups”, “Cue mix Masters”, or any kind of Summing Point you can think of.
Cue feeds are traditionally pre fader sends from the channels, as you may wish to have subtly different mixes going to each cue, and be distinctly different to the mix that is on the master buss. Ardour already allows for this, including the ability to add a pre-fader send to each channel with the send level matching that of the channels main fader, ie instant starting point for your cue mix. You create a bus, then from the right click menu of the Aux button select “add pre fader send to each channel”, then from the same menu select “copy channel fader levels to sends”.
As Paul said there is still work to do here, but a lot of what you want is already there.