If they do the diving update in June/July/August, they could promote it as the northern hemisphere players doing leisurely summer dives for shrimps and clams while southern hemisphere get to test their mettle by diving for cold-water crabs and lobsters. I do agree that a lot of these discussions are are NH-centric, but the actual updates are largely either things that are available year round or holidays that occur on a specific date regardless of what hemisphere you're in.
Christmas is still in December in the southern hemisphere.
I think this is the key people are forgetting. The actual holidays still occur on the same day they do in the northern hemisphere. Instead of a summer update it could be more a split update that introduces new furniture/clothing that matches either season (with it being seasonal locked). Diving is universal and so is a Nook’s Cranny update so nothing really special there.