Here's one possible format I could see working based off what's already been announced and the schedule.
Week 1 - Top 24
We already know it's just a one-week ordeal for the Top 24 rounds. The S18 returnees won't be involved this week (which is perhaps why they haven't announced it yet - maybe they're waiting until the end of the episode to announce it for the ~shock factor~)
Week 2 - Top 16
We also know we're going down a Top 16. What we thought was that they'd announce the Top 16 on Sunday's show and have them all perform again, with Monday's show being the Top 10 Reveal + 6 wildcard performances to make it a Top 12. However... I wonder if Sunday's show will actually cut down the current Top 24 to a Top 10, and the judges will choose 2 wildcards from that cast to get to 12. Then the 10 returnees will perform, and there's live voting from America for four of them to advance to make it our Top 16.
Monday's show will be a Top 16 performance show with the 12 from the current cast + the four returnees. Live voting once again (as I've pointed out before, ABC has already done live voting on Mondays with DWTS, so it's just as possible we'll see it happen here too.) At the end of the show, four from the Top 16 will be cut regardless of what season they're from to get us our Top 12 going into week 3.
Week 3 - Top 12
Core finalists, regardless of season make-up. The season will proceed as normal from here.
tl;dr:
Week 1 / Top 24 - S19 cast only
Week 2, Sunday - Top 10 from S19 cast revealed, judges pick 2 wildcards. 10 returning contestants perform and 4 advance to make it a Top 16.
Week 2, Monday - Top 16 perform and 4 get eliminated, regardless of season.
Week 3 onwards - Top 12 performance show, and the season proceeds as normal.
It's a bit of a drastic cut to lose even more of the S19 cast, but given they had Alyssa return in S17, the twist for someone from S17 to get a free pass to Hollywood and now the S18 returnees, ABC seems far less concerned about multiple-season Idols than Fox did.