will varvara get a white beast transformation since she is blessed by seiros?
Sort of! She probably won't turn into a white beast, but the idea does influence some of the plot for Sapphire Haze.
I was thinking it would be good if - rather than feeling like a random boss for Silver Snow - there was later story significance, and if another route tied into that boss, it might be fun!
So...
During the late game, Rhea will discover (via an Agarthan experiment or ancient historical document or something) that dragon/ Nabatean blood may cause some catastrophic event should the dragon/ Nabatean loose their mind. Classic FE dragon stuff.
She comes to King Dimitri and Archbishop Byleth in shock requesting that Princess Varvara join them immediately. There she would share the news, terrified of their reactions, and for the wellbeing of those who had been under her care. Rhea had been using her blood to heal people, for the most part, so hearing that she put them in imminent danger will really impact her - we never see her reaction in Silver Snow, because before it happens it seems to be unheard of. (Seteth has no ideas about what's happening)
Varvara was given Rhea's blood as a child, when she was injured playing in the forest near the castle. She was taken to the monastery to be taken care of and Rhea eventually had to give her a healing transfusion. The crest of Serios only manifested in crestless people she healed, so the crest would have no negative or positive effect on someone who already bore a crest (other than healing), and after getting the OK from the Archbishop, Rhea gave the child some blood.
Anyway, back to present day, and King Dimitri requests a meeting with the Blue Lions and child units, to brainstorm what might help Rhea from loosing control and causing a catastrophe. One of the child units who has experience with Agarthans (so excited to reveal them later but alas, I have an order I want to do them all in!) as well as Sylvain who has a good grasp on magic and technology (three hopes) suggest a few things, and eventually Flayn suggests finding a mythical Dragon Stone. Or maybe making one. I haven't figured that bit out yet.
Eventually after the Dragon Stone is located (or made), there's a timed turn battle where Avery has to get either the heavily guarded Dragon Stone or get a magical item to finish the artificial Dragon Stone (again will figure that out later). Rhea has been getting progressively sicker. She insisted on coming so that if they get the stone they don't have to transport it all the way back.
Varvara at this point will also be getting sick, and so now one of the strongest units is unusable, there's a turn based timer, and there's a boss battle. It should be a tense/ fun gameplay experience.
Varvara: I'm Ok, just get Rhea the help she needs, Avery.
Avery: Don't turn into a dragon, Varya, Promise me. Rhea said we wouldn't be able to get you back if... sniffle.
Varvara: Hey, look at me. I know you can do this. You've never let me down before. Not when it counts.
Varvara: Besides, who will tease you about that Tassets-less armour if I die?
Avery: Another tasset pun?! Kinda glad you're not going with me!
Avery: But in all seriousness, stay safe.