Is the control panel a good height for a racing wheel? Should the pedal be permanent, or pulled out along with the panel? Do I need separate panel for shmups and rotary games? Should I put a spinner with the trigger stick to play tron, or or double it up as a racing wheel? Can I split my analog inputs? Do I have enough clearance for my hand on a trackball panel, and If I angle the monitor for more hand clearance, will it be too angled to use lightguns? Etc, etc. Unfortunately, the way most people solve these questions is to just buy the goods upfront.
Good points here, when I get above 5 panels I may start to appreciate some of these concerns a bit more

Although the panel attachment will be at a given height at the sides, there will be nothing underneath the central 90% of the control area. So if needed the steering wheel panel can sink down in the middle to get the appropriate height.
I think there will be a permanent pedal box. How many pedals do I need?
I haven't though about shooters, they don't work on PC monitors yet do they? Would love to play point blank again.