I don't think that this would have changed the character dynamic between the scoobie nearly as much as people think, what it would have done is to seriously complicate the plot for certain episodes and events. The age gap alone would mean they probably would not spend all that much time together, 5-6 years would mean they simply would not have that much in common. However Joyce would probably expect Buffy to help out with Dawn from time to time, and that would have complicated the over all plot from time to time. I think the one dynamic that might have radically shifted would have been the relationship between Buffy and Faith, particularly early on, because I think that this was a pseudo sibling dynamic. At one point in Faith, Hope and Trick Buffy even remarks on "her new little sister."
btw Their was a series of graphic novels that came out shortly after Season 7 ended that kind fills in a lot of the gaps that happen prior to the first season and continues through at least part of season 1, and Dawn exists in this version and it is interesting to note some of the things that occur because of this fact. For example Dawn was the reason why Buffy was committed to an institution following the events at Hemry, this was mentioned in Normal Again in season 6.
