Thinking about the current plotline a little more...
I think that it could've been more effective if DMU and ONE had been split into a pair of sets each. Let's call them, I dunno... Dominaria United/Dominaria Besieged and ONE/TWO (I can't think of a good name for the second ONE set, so sue me
)
1) Elspeth gets tipped off in alt!New Capenna that Sheoldred is planning on invading Dominaria (because that's what Daddy Yawgmoth would've wanted).
2) DMU is spent with a bunch of planeswalkers running around trying to get Dominaria ready. Sheoldred's in the set... and so is Ajani, Sleeper Agent! They've had a mole in their midst the whole time.
3) Dominaria Besieged focuses on the actual invasion, as a big "hey, this is what it'd look like if New Phyrexia got their way!". This is the point where they hatch the mad plan to make their own sylex and travel back in time to see how it works (going back to BRO).
4) BRO happens. I liked BRO - it can stay the same.
5) ONE is the initial attempt to get the sylex to Phyrexia to blow stuff up. Assuming I can fiddle with the planeswalker line-up, they send in Elspeth (who meets up with Koth), Nissa and Vraska. This attempt to get the Sylex to the heart of Phyrexia fails, and the invasion force gets scattered. Elspeth escapes being compleated and gets back to Koth, while the other two get compleated (with compleated Nissa being the key to Realmbreaker working).
6) TWO is about Jace and Nahiri staging a rescue mission. Jace gets compleated by Vraska, but Nahiri meets up with Koth and Elspeth. Progress on Realmbreaker is mentioned and the possibility of the Sylex wrecking everything if it is comple(a)ted is discussed, but the decision is ultimately reached that they'd try to get the Sylex in position before it hits the "connect the multiverse" point.
Importantly, the planeswalkers are split here — Koth is adamant that no cost is too great to defeat Phyrexia, Nahiri is all "actually, I've recently learned that mass genocide is a
bad thing — containment might be possible?", and Elspeth is on the fence. They get to the center of Phyrexia, see that Realmbreaker is activated... and Koth uses the Sylex anyway, while Nahiri tries to stop him. Elspeth grabs it and BAMFs away, leaving Nahiri alone and hopeless.
BOOM. IT'S MOM TIME. TIME FOR MOM.
...
The exact planeswalkers they send can vary (I'd definitely keep Elspeth, Jace, Koth, Nissa, and Vraska, if only because they cover the full
, but also because they have the most interesting story beats), but it honestly flows better if we don't end up with two planeswalker cycles in a set that really shouldn't be focused on the planeswalkers.
This would also let them play with the factions on Phyrexia a bit more — ONE could be mostly just "here's the status quo", while TWO would be the set where Urabrask makes his bid to topple Elesh Norn.
EDIT: I cut the Emperor, Kaito, Lucca, and Tyvar because I'm not sure that there's a compelling reason for them to be in the set? We can stick the Emperor and Kaito back in one of the DMU sets, and Tyvar and Lucca can wait for MOM.