While very early still todays weeklymtg had some info thats lore related;
First and minor: The marketing art shows Karn, Liliana and Ajani which likely points to them being in the storyline/on plane (and planeswalker balance fits a B and WG walkers) and Jodah and Shanna features as well. Ajani was a safe guess as he hasn't been featured since war of the spark and Elspeth said he was on Dominaria but Liliana is a bit of surprise as it seems someone pulled her out of her nice teaching job.
Bigger New: Dominaria United will be the start of a storyline that will take place across several sets. While they couldn't give details, pretty clear this when the Gatewatch and New Phyrexia will begin their war.
They also give out the codename for the next few set with the 2021 Q2 date being a big note as list is "Codename Marathon" and with it "Codename Marathon Epilogue".