Marble Madness. While most times you might be rolling the ball at a diagonal or down.. there are times when you roll it sideways. There are also other trackball games where you would roll it sideways.. and future games, in which you might want to play.. which also utilize sideway rolling.
Many people dial up the trackball sensitivity, and so a small amount of roll.. will result in a high character movement in mame. The problem with this... is that its not correct. It makes the games easier than intended.. too easy. You also lose fine control, like a razor vs a shovel... so in games which require both precision, as well as speed.. you will suffer bad results on the precision control areas / aspects.
Its also another aspect of mame that Should be attempted to document and clarify. Documenting the encoder wheel diameter, trackballs roller diameter, ball size, and optical board signal rates.. and then with use of formulas, one could make a program which calibrates the controller to match the correct input ranges.
It might even be easier than that. Even if its a simple rough estimate for temp sake... by using several on the fly created comparison videos of a real machine, and a mame machine.
As for the buttons on the 3 / 4 players. Do a mock up mounting on a few sheets of cardboard stacked together. Make sure that the angle of the buttons feels comfortable with the stance you will need to take. (Utilize the stick at the same time as well, to make sure of distances and help with accurate results in the button tests)
Also, I agree with the recent post. If you are planning to play console games that use more than 4 buttons.. and you do not mind making the control panel a little wider.. do it. This is about your happiness.. not trying to please the cabnazi's. Theres never a way to please everyone else... so dont bother. Do what pleases you, for you.
Note, that if you lose your 4ways.. games like MsPacman wont play very good. Physical restriction is a completely different experience... and its far better than spacer flip sticks or virtual restriction. Nothing comes close. If you dont play 4way games much, and do not care about losing lives quicker.. then its another story.
If you want to play games like Asteroids Deluxe.. you may consider putting at lease one real leafswitch button on there. These are great for rapidfire games, as you wont get as fatigued as when you try with microswitches. The new hybrid micro-leafs are better than micros for less effort + reaction speed.. but they still wont feel nor be less fatiguing than a real classic leafswitch.
If you want to play Robotron.. you would want two real wico 8 way leafswitches. IMO, nothing else can match their performance and feel, for this game. Leaf sticks also feel better for older classics games. They do not work very good for fighters however.
A spinner is a must IMO. Otherwise you lose out on some excellent games. You can also use a spinner to control driving games. You can put a pedal set as the base of the cabinet too. Also, you can mount a spinner horizontally, like a volume knob... (in the front of the CP face) though, I do not think anyone here has done so yet.
A trigger stick or button topped stick, would complement the spinner... and allow many of the dual controller games, such as Tron, and Mad Planets... among many more. Mad Planets alone is worth it, IMO.
If you play Virtual Pinball.. you may want to add an analog plunger, and internal bump sensor.
If you want some cool effect.. pop a few bass-shakers in there. Similar to a subwoofer.. but they do not move air. Instead, they just take the sounds, and turn the mid to lower bass / sub frequencies into direct vibrations.
Comfort Ideas.. One thing I plan on doing, is adding a set of fans that blow air over the players hands.. to keep them sweat free. Possible under CP using angle drilled holes.. Possible small protruding vent hoods. And Or possibly in the panels backplate.. which could have a row of them.