Realistically: I foresee multimode 520s as the next big thing.
I don't really think 475nm diodes nor 490nm diodes will become common or cheap. there's no practical use for diodes of those wavelengths.
Idealistically: Yellow diodes may be the next type of diode to poineer for better color balance... and I really like the 480nm color range... I'll get my hands on a nice diode someday.