When it comes to release dates, I speculate 10-11 months between major wave releases (Halloween doesn’t really count as it’s usually only a few Pokemon). In this case, all of Johto was released in February 2017. The first wave of 50 Pokemon from Hoenn was in early December 2017. The first major wave of 20+ Pokemon from Sinnoh was in October 2018. If that trend continues, we could be seeing an August release, but I can see it being pushed to September based on the events going on in the summer.
All the Legendaries outside of Regigigas are technically out now, which I’m sure we will get at least 4 weeks dedidated to that. That means we still have over 2 months worth of summer to fill. We could see rotating Lake Trio, which would be the safest bet (and for me desirable just to get the entries) but we could get a LOT of returning Legendaries to fill the void. We could see an increased presence of special Raid Days as well. Niantic is going to have to keep us very busy this summer or else Pokemon GO will suffer greatly.
They COULD move up Gen 5’s release to mid summer, but we might have already seen data for that already had that been the case, so it doesn’t seem likely that would happen. I also put these speculations out there to help think about raid battle teams for each Legendary that will cross our path. As I’ve seen so far, we have a fair few Trainers here on the forums that are very unfamiliar with Unova, so giving them a heads up should help them along with preparations and expectations.