Advice: Never Curve a Button Layout
1) Most everyones keyboard keys are in a straight line, and your fingers remain bent, to get better leverage to press them
2) Everyones finger lengths are different, so even if you had the finger strength to press a button from a completely flat-hand finger spread...
each person that tried to fit your layout with their handshape... might have alignment issues... where their fingers dont fit your layout.
3) I can only assume that the Japanese cabinets can partially get away with this, because they use Flat or convex buttons, with low-activation force switches.
I personally still cant fathom how anyone can use a flat hand to press these buttons for any extended period of time. You have far less leverage force, and it
would be incredibly fatiguing, even with low activation force switches.
I also prefer the US standard Concave buttons. The shape funnels your fingers into the center of the button, and helps prevent your fingers
from easily sliding out of them. Its also much more Comfy.
4) As Badmouth said.. Its too easy to Lose track of which buttons that your fingers are on / over. You might accidentally also press the side edges of a button,
due to misalignment. With a Straight Layout, your fingers are always in-line with all of the buttons, no matter what size/shape your hand/fingers are.
While there are some Ergonomic Keyboards with a curve... they tend to be a very shallow curve, and your fingers remain bent while operating them.
With these large sized arcade buttons, the spacing is too large... so the curve becomes much too steep.
The amount of space you might save from curved layouts, is very Minimal at best, and really isnt worth the control issues.
The only exception, is a lower-thumb button... like the Run button in MK3. Since its only a single button that isnt pressed often, it works fine as
something you can quickly reach for, with an extended thumb.
The only other thing Id personally do, with a button layout.. is to have at least One, Classic Leafswitch button.
These are excellent for Rapidfire shooters, as they are much less fatiguing for high-speed repeat-firing games (like Halley's Comet).
If you only have 4 buttons, a diamond shape isnt that bad... but honestly, Id still always stick to a 2x2 straight line.
Diagonal button layouts are only really good for Gamepads, because you can roll your thumb on them like a d-pad.
Even then... I still preferred the Genesis straight line button layout.
Large arcade buttons are different, because you cant easily switch from horizontal resting, to the top and bottom buttons.
When the buttons are in two straight line rows, its easily to slide both fingers from one set of buttons, to the other set.