If you go with an RGB back light it could be controlled with an Atmel ATtiny45, a trimpot and some simple code like Paul Boardman uses here
It would not be that hard to write the code so that the ATtiny45 changed the backlight color via PWM input from an Arduino either.
As for LCD form factors I would like to see a 24x4 or even a 30x2 or 30x4.