I think a 'crystal clear' button would be cool - as you could light it any color using lights or led's. (even changing the colors on the fly : )
Slightly frosted look might be interesting...
A matching clear 8way wico balltop handle would rock... that and a happs competition bat handle too.
Making them microswitch and leaf switch capable would be cool. Maybe thru the use of an adapter if needed.
I also think it might be nice to reduce the footprint a bit...maybe just a touch bigger than 3/4ths normal size. That would allow for less cramped panels, better mini panels, easier to reach buttons (less finger spread), easier for people with small hands...ect.
A few 1/4th size buttons would be nice for things like emu controls, coin in, start buttons, ect.