I am predicting that the Hero Academy will enter beta by March 2020 earliest.
The sneak peek for 2020 announcement states that Season 3 is coming early 2020, so I expect to see that in beta soon. Possibly even next week. Likely taking priority over HA.
EDIT: yikes, we just got S3 in beta today! 
Same with the seasonal events, or at least the Springvale one. They also mention they want to update them this year. January doesn’t have many days left, and it would be last minute to put the event in beta for March. So I am thinking that Springvale revamped is also taking priority, and we’ll see that in beta probably in February.
Maybe I’m being too pessimistic or optimistic depending on your perspective – but for the above reasons (S3 and revamped Springvale) that’s why I think Hero Academy is going to enter beta in March 2020 earliest.
remember, all this is speculation on my part