Actually there is a pattern and you are right about Tarlak and Mitsuko.
There are 10 atlantis 5☆ heroes, they are divided into 5 groups:
- Ursena + Inari appear in both May 2019 & October 2019
- Tarlak + Mitsuko in June 2019
- Kageburado + Atomos in July 2019
- Misandra + Poseidon in August 2019
- Mok-Arr + Ariel in September 2019
By that pattern/cycle, November 2019 shall be Tarlak + Mitsuko, followed by:
- Kageburado + Atomos in December 2019
- Misandra + Poseidon in January 2020
- Mok-Arr + Ariel in February 2020