i doubt this is a new idea since whatever anyone thinks is new has been done several times before but just doesnt seem to pop up with the search feature.. 
but i designed this layout for my buttons as a somewhat non-traditional approach to the combination of a capcom layout with a neogeo layout. and if it happens to be original, it was my idea! the spacing is the standard 1.5" from center to center between adjacent buttons.
It's not original, I've seen this layout used on cocktails before, probably because it doesn't need much space. However it's still a good idea! I'd probably use it if I were building a cocktail cab.
With regards to the number of buttons needed, another reason for using a 7th button (apart from Neo-Geo fighters) is that it's handy for reverse on Defender/Stargate.
However, I really can't think of a good reason for an 8th button. For a start Dreamcast games generally only use up to 6 buttons. Theoretically Playstation and X-Box games can make use of 8 buttons but how many games actually do? And how many of those games are suitable for using with arcade controls? Not many I suspect.
My next panel will be designed mainly for DC games and I intend to use a 2*3 layout in an arc because that is the layout that most Naomi cabs (based on DC hardware) seem to use.
I have a slightly off topic question that I'm sure has been asked many before but I've had no luck with the board's search function. Please can someone tell me how to wire up an Ipac to a 2*3 Capcom layout so that the Ipac does not have to be reprogrammed (i.e. the jumper is set to the MAME defaults)?
Thanks in advance