I hope that the city decides to make it as much of an even split as possible. I feel like they'll end up saying Skyline Middle (which is essentially the eastern half of Harrisonburg) goes to Rocktown, and Thomas Harrison Middle (west side) goes to Harrisonburg. Heck, Harrisonburg High and Thomas Harrison are both on the far west tip of the city limits already. Rocktown is about as far south in the city as you can go, so it wouldn't be the most ideal location, but it makes the most sense.
I think, with the right coaching hires and proper investment, both schools could compete well athletically in Class 3. Like One Man said, playing a bunch of Loudoun County Class 5 Schools along with Albemarle, Patrick Henry, and William Fleming (who all have great athletic programs) will be very different than splitting and playing in Region 3C. This has nothing to do with any sort of superiority, 3C has some fantastic athletes and schools (Spotswood and Western Albemarle are two crown jewels of the commonwealth) but it would just be a very different look which I think will help.
I would expect both schools to immediately compete in soccer. Hopefully, the split of the schools will also help the football programs gain some traction. There is athletic talent in Harrisonburg. The basketball program at Rocktown may need some time to get moving but Burgess has coached some fantastic teams at HHS in the past. As an example, I remember in 2020 when HHS boys basketball won the valley district, beat Spotswood twice and had one of the best seasons in the valley then lost in the first round of the playoffs to an also really impressive Fleming team. Obviously splitting would divide up the talent but it would be nice to not play Fleming or PH or Albemarle in the first round of the playoffs anymore.
The physical building of the new high school looks incredible. Haven't been inside but the athletic fields look great, it's on a beautiful hill overlooking 81 and the building is top notch.
Just as a final little tidbit, last year Harrisonburg High School had an enrollment of 1953. 29% white, 11% black, 53% hispanic, 4% biracial, 2% asian. The city is super diverse and I have loved living up here for the past (almost) four years!