Seinfeld Is Unfunny: Timbaland perfected his electronic-sounding production style on One in a Million, and it's a staple in R&B today.
What Could Have Been: Trent Reznor - yes, thatTrent Reznor - was asked to contribute to Aaliyah, but had to pull out due to scheduling conflicts. He hasn't talked much about it since, but it would still have been interesting to hear some industrial R&B.
There's also the much more obvious case of What Could Have Been. A lot of the reviews for her third album said that while it was very good on it's own, it also shows the enormous potential she had.