1280x720, right?  I'll try and have a look at recomposing in that ratio - might work out after all.  I'm planning to use the 
Mr Do cabinet images since they're so big and beautiful - as you can see from that last screenshot.  There could be a way of working that into all that width you've got there somehow.  I can't promise anything but I will look into it 

  I had a quick play with a 1280x720 variant after you originally posted, and it turned out crap - but most new designs suck until you sleep on them for a week or so and then work some more.
My desktop machine runs in 16:10 so a widescreen skin would be sort of handy for me anyway.  I can imagine a few others in the same boat, even if cabs are generally 4:3. 
Regards the tree thing - I'm not sure if I'll use Tree.  I have played with it a little.  Was thinking the tree views of this skin would basically be that side menu centred and bigger, with the icons larger too - then when you drill down into a specific emulator, the screen shown replaces that view and you can choose a ROM.  So basically, 
stealing horribly from this concept.
The thing with trees though is that they are a pain to set up (lots of screens to render), and you have to move up and down them to change your emulator.  Without a tree, you can just map a button or stick to swap the emulator.  Trees are certainly more user/newbie friendly though so I'd consider them for a cab you want to leave unattended.  Personal preference really.
I'm thinking for this one that I'll just map either a "shifted" joystick up/down to change emulators, or buttons 3 and 4 or something. Each emulator then needs its own variant of the skin with the correct "button" highlighted.