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.