Perhaps Rachael is to outlive Deckard? As she is, after all, an advanced Nexus series. Knowing that time is robbing her of him she must crack the code of the key to immortality on the back of Tyrell's findings. Thus learning how futile man's pursuit for permanence truly is.
There is also the possibility that assuming she functions on the same biological level she could procreate. Imagine that replicants replicating?
Ooh the speculation. Well pepper me in glee today.