We’ve been told that each Atlantis portal will have only two S2 5* available. That’s certainly what we’ve seen: in August we had Misandra and Tarlak. In September, Tarlak and Mok-Arr. I expect that in October we’ll have Mok-Arr and a new S2 5*. I’m assuming that, once they’ve cycled in all the new 5*, they’ll start the cycle over again (so putting the final 5* to be released with Misandra), but that’s a long time to wait.
I missed Misandra in August but did get Tarlak. I didn’t do any Atlantis summons last month, anticipating that I can draw for Mok-Arr this month without risk of getting a duplicate Tarlak (and I already have Ares and Alberich from their original release).
If they keep up the pace of 5* heroes coming in, we get one S2 5* per two S2 provinces. That implies there could be 12 new S2 5*, which also implies that we won’t see Misandra or Tarlak again until next August. That’s an eternity for an on-line game.
EDIT: we haven’t seen any new S2 4* heroes in beta, but we do have three candidate S2 5*. I’m guessing that there are no additional 3* or 4* S2 heroes planned, but it’s just a guess.