Always disappointed me how they left the Data story in part 1, could have expanded on that nicely. I loved the abstract dream sequences, part 1 is much better for it.
They did follow up on it, in the episode "Phatasms" in season 7, when Data started having nightmares. I mean, what left was there to say on the topic of his dreaming after "Birthright Part 1"? I think it pretty well covers all the highlights and anything additional in Part 2 would simply be retreading the same ground. The most logical follow-up to Data dreaming is Data having nightmares and they did, just one season later.