I'm kinda thinking of something that would either be "perfectly" sized, or something that can be easily cut down. Me, myself, personally - I'd never buy anything like an X-Arcade, or Mas Stick, because I build those. I would however buy a used full sized arcade machine (no multi game), because I think they are cool but I don't have the place to put it or the money.
What I am thinking of is a kit that would ship unassembled, but would have all the necessary equipment, such as joysticks and buttons. It would have a clear plastic sheet covering the panel under which people can put whatever graphics they want (though I haven't investigated whether Kinko's or whoever produces 24" printouts just yet). It would still allow anyone to build the complete cabinet themselves, but the difficult machining of the top panel would be taken cared of, all the parts would fit, all the parts would be included from joystick/buttons -> wiring harness -> interface. And the complete kit would costs less than the sum of all parts bought separately.
On that note, maybe I'd leave out the panel and just sell the joystick/buttons, interface, and wiring harness. I don't know yet I'm just toying with the idea.
I'm thinking that I would want to produce a variety of different panels, custom made to cust spec.