My plan is to have two cabinets (for now). I hate crowded frankenpanels and I want to be able to play just about everything. One of the cabs will be a 2 player, 8 button/ 8 way joystick per player cab for fighting games, SNES and later console games (just ones good for a cab of course), and just any game that needs more than 3 buttons and is horizontal, and uses an 8 way. This cab will also have 2 "modern" guns, ala Time Crisis and the like.
The second cab will be my "classic" cab. It will be a modified dual monitor PlayChoice-10. It will also be 2 players. The botom monitor will be horizonal and the top one veritcal. I'm doing it this way for a few reasons. The main reason is that I always liked the PlayChoice 10 and Punch-Out and want to play those as close as possible to the real thing as I can. The top screen doesn't contain the gameplay, and mame is able to rotate the horizontal image and it looks just fine for this "non essential" screen for just a few games and PC10 instructions. Of course for other games, they would route to the appropriate screen. I don't want the cab to differ too much on the outside from the original, but it will have two "classic" (Nintendo, Lethal Enforcers) guns, rather than just one. I am also going to add one extra button under the B and A for a total of 3 buttons and one stick per player. This cab will have U360s for maximum compatibilty. I plan on doing (or having done) the rotary mod for Ikari Warriors. I really don't want the top fire, since I want the sticks to at least mostly look like original Nintendo sticks. I'm hoping 3 buttons should be enough for any games that would use a top-fire.
These two cabs will also "link up" for playing dual screen Nintendo Vs games, 4 player games (TMNT, Simpsons, etc) and four joystick games like Smash TV and Total Carnage.
As an add-on to this, I plan on making this thing extra panel to sit on top of my "classic" cab panel as necessary:

Excuse the crappy (very badly) MS-painted mock up of a Rampart control panel. It will also have "pinball" buttons and possibly a plunger. The five buttons are for Toobin (and other things of course).
I can also have two "flight sticks" with suction cups to attach to this for Tron (Forgotten Worlds maybe) and whatever else.
The spinners I'm planning on using would be 360 degree push/pull so they work for every (??) spinner and 360 top view driving games.
The PlayChoice panel would also be used "as a whole" for games like Defender (2P buttons for Player 1) and Robotron.
What do people think of my crazy ideas? The only thing missing is racing games (Outrun, Crusi'n and the like). Whenever I get room for a sit down cockpit cab..... (also for Steel Battalion on Xbox).
Not sure how I'm going to handle 6 player X-Men... (probably a PS3 and widecreen TV, outside of these two cabs)