Space and time. Though, If I were honest with myself, I'd really only be interested in building a driving cab (and maybe a video pinball) in addition to my buffet project. The buffet can play anything I want, plus windows games, plus Frets on Fire, Visual pinball, future pin, stepmania (a DDR clone), etc, plus (eventually) gun games.
And it completely disappears when I'm not playing.
A pin would be great, but they're big, and I haven't figured out any way to possibly conceal one as something else. Same with a driving cab, only it'd be even bigger.