I've seen the idea to have Rhea as the lord in Silver Snow and, though I didn' read/watch anything regarding it, I didn't think it would work, but upon further thinking, I realized it could work, and decided to that idea one step further.
What if Rhea's role in each route was different?
Verdant Wind/ Crimson Flower: I'd keep Rhea's roles in these routes the same, CF because it's already different and VW because one can stay the same.
Silver Snow: Again, I haven't read anything regarding the idea, but I'd imagine my ideas are very similar to others. So each route follows a different ideal, in CF it's 'everyone should be equal,' in AM it's 'we forever fight for the people,' and in VW it's 'everyone should be united'. When rewatching the cutscenes in SS, it was hard to find an ideal to attach to it, but there is one scene where Seteth and Flayn (of course Byleth is there, but doesn't say anything) and basically say 'no matter the cause war is not justified', and when considering that Byleth becomes the ruler at the end, I think the ideal is 'end all conflict, then fix the world,' that would leave room for interpretation since Byleth represents the player. So, keeping that in mind, I think Rhea's role in SS should've been an example of how NOT to do that; we know that after the War of Heroes, Rhea came up with lies regarding Nemesis and the 10 Elites and banned certain machines (see the shadow library) in order to maintain the idea of Sothis' divinity. The problem is that TWSITD exploited this to provoke Edelgard into turning against the church. SS should have used Rhea to make that apparent, and it would make the final battle against her beserk form symbolic as it would've represented the end of an era.
Azure Moon: this one is going to be small by comparison; just have Rhea die in chapter 12, lets say Thales does it. The most common complaint I've seen for AM is the absence of Rhea, which is because of how they end each route. This would be a non-issue if she was dead. It would make it clearer sooner that certain questions won't be answered because the answers were lost with Rhea, and the knights still have a motive to follow her successor.
In order to make these differences possible in each route, I think chapter 12 should be different for BL and GD. Basically, let's say the knights are the defense, BLs are at the front lines (because of Dimitri) and GDs attack from behind, possibly encountering Aurendel's reinforcements. So in SS Byleth protects Rhea, In GD he stops Aurendel/Thales from killing her, but fails to stop Edelgard from capturing her, and in BL, he isn't there to stop Thales from killing her.
How does this idea sound, or am I talking too much about what could've been?