My thoughts:
- There have been rumors and theories of Ant-Man 3 setting up a next gen Young Avengers.
- The Ant-Family has always been important to those, with Cassie a core Young Avenger, Scott having led the Future Foundation and Hank having led Avengers Academy, so a lot of material to pull from.
- Besides Cassie, we know Kate Bishop will be in Hawkeye, Wiccan and Speed are pretty much guaranteed to be set up in WandaVision, America Chavez has been heavily rumored for Doctor Strange 2
- Kang has a major role in Young Avengers and the whole Iron Lad plot
- Kang would probably show up after Endgame timetravel shenanigans which used Ant-Man related tech.
- Kang has major ties with the F4 and taking parts of Scott's Future Foundation period as inspiration could tie in with that nicely.
Makes sense to have him there imo.