I think the main advancement has been fitting multiple drivers into earphone that still fit in your ears. However, the sound doesn't necessarily improve with more drivers. However, if etymotic took the er4s and added a sub bass driver that would be a pretty darn amazing feat. If that would allow the earphones to not struggle in any one area and thus allow them to fine tune the treble even further (very slight), you would have the perfect iem. Alas, I have yet to hear such an IEM. I'm speaking of universals. I have no desire to spend a lot of money on a custom iem i can't hear first and can't know how it will ultimately feel and sound. And I've yet to read a review from someone who loves the er4s that describes any of them sounding better. I think the technology exists to make a better iem, but no one has done it yet.
I don't think Etymotic would stray from the Olive-Welti diffuse field target curve and I wouldn't want them too either. Matching that target curve has been their niche and they do it very well. I look forward to the day they release something other than what my grandma would want.