Is there any ETA for Borons and Splits (nations from previous games)?

The Split will be added with the first DLC, which is currently scheduled for release after update 3.0. There is no release date available yet, my personal guess is not before fall 2019.

Borons... who knows. Maybe the 2nd DLC (2020).

More info here:
https://www.gog.com/forum/x_series/news_about_upcoming_updates_20_25_and_30_and_the_first_dlc

There are gates in Heretic's End which in the past connected to the Sol system. So the Terrans might come back as well. In contrast to Borons, you don't need fancy animations for them as they are humans just like the Argons.