I'd say the Zamasu Arc as a whole would need a rewrite from the ground up due to the absolute mess the Time Rings cause to the timelines being coherent. If only limiting it to the latter half though, then that huge flaw remaining is just a sacrifice we'll have to make.
- Provide more time and steps to Zamasu's heel turn.
- Make Goku Black's identity something better than Ginyu 2.0/poor man's Tullece. I'd probably go with making Black similar to Mira, being an artificially created warrior made with the help of Demon Realm science/magic using Goku's DNA from Zamasu travelling back in time, hence why Black didn't have access to many of Goku's techniques and could still gain Zenkais.
- Have Goku attempt to use the Zen-Oh button when injured by Zamasu, but have it destroyed. All it could amount to was a literal deus ex machina as well as bring in another Zen-Oh without good reason.
- Follow the manga version of events up until Vegetto diffuses with some major exceptions; Goku doesn't bring the wrong seal for the Mafuba and have Vegeta train with Trunks in the Rosat (also establish they can go in for more than 2 days due to Dende and Popo upgrading the room), with it being established that the training helped Trunks tap into more of his hybrid potential and reach the level of the SSBs with SS2 alone, as well as be on the cusp of unlocking something more.
- If we are to include PSSB due to SSB's stamina drain making it a pretty big Chekhov's Gun, have it established that both Saiyans sensed the ability to achieve it when their insight was combined as Vegetto, but have Zamasu injure Vegeta too much for him to achieve it himself.
- Trunks achieves SSJ Ikari like in the anime and, after having watched and understood the basics of the Mafuba, manages to combine it with his sword to seal Zamasu in the sword as a callback to DBZ Movie 13. Trunks saving his future is a necessary plot thread when he should've been the central character of the arc.
- When the cast bring up using the Super Dragon Balls to revive all those killed by Zamasu in the other timeline, Beerus tells them it'd be impossible to do without Zen-Oh or the Daishinkan finding out and erasing both versions of Universe 7 as punishment for using time travel. As such, the repercussions of the arc remain intact, as does the theme of not relying on the powers of the Gods. Trunks and Mai try to time travel to a time before Black appeared, but the damage that the fight provided to the time machine causes it to act strangely before disappearing, leaving it ambiguous at to whether or not they travelled to the right time period successfully.