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.
She was also the Wachowskis' first choice for the role of Zee in The Matrix Reloaded and The Matrix Revolutions, which would have been her big acting break; after her death, the role went to Nona Gaye instead.