I have lots of ideas for making a useful debugger/monitor, because I especially hate the VICE monitor as it is right now.
I much prefer the Power64 debugger. It has nice commands to dump the state of the VIC/SID/CIA in readable form.
It also has a powerful 'find address' command, which finds an address being used in absolute and zp addressing modes,
and if it's used as a branch target. It's also much more like traditional monitors on the C64, so I don't have to
consult the help that much. It lacks watchpoints though.
There's an Apple II emulator called Virtual][, which has a nice little debugger, where you could get some ideas from.
See here:
http://www.xs4all.nl/~gp/VirtualII/VirtualIIHelp/InspectorHelp/inspector_help.htmlAlthough it does lots of things in the UI wrong, it's still very good.