I have a 5:4 LCD monitor that I wasn't using. It's 17 or 18 inches but it sort of has dimensions of a 19" because of the outside edge. Ideally I'd want a larger screen but I do admire the 5:4 aspect ratio since it's classic.
I like Galaga a lot, but I know I'm not building a vertical screen to replicate that. I don't have a ton of good arcades in my area so it's hard to find retro stuff to play.
I also like pinball so I'm adding side buttons to the CP like the arcadecab tut. Big fan of Street Fighter as well as other fighting games if I can find someone to play with.
One of my goals is to actually add some of the less common controls to my CP to play some of the games I couldn't on a normal controller, including Spinner and Trackball.
I've mostly been a console gamer all my life so building a cabinet is my way of branching out