WOTC announced all the main, standard-legal sets for 2020. Naturally there will be supplementary products like Masters Sets/Battlebond/Conspiracy/etc, but we know what the standard sets will be!
Winter (Q1): Theros: Beyond Death
Spring (Q2): Ikoria: Lair of Behemoths
Summer (Q3): Core 2021 (w/Teferi theme - probably similar to Chandra theme in Core 2020)
Fall (Q4): Zendikar Rising
What we know:
-Theros Beyond Death will focus on Theros' underworld, and we will PROBABLY see Elspeth again.
-Ikora is a monster-focused set, and apparently you can some how "make your own monsters." I'm not sure what that means, maybe it'll be something like those creatures in Unstable that could be combined?
-Zendikar won't be focused on Eldrazi. Remember, they left for Innistrad during Shadows Over Innistrad. It will be more like the Zendikar from the set - a Dungeon-and-Dragons, adventure-based fantasy plane.
Also, for anyone interested, I talked about it a bit in this video, though it's pretty much 6 minutes of the information you just read.