The first time I heard the podcast about how GW comes up with the rules I really like how they attempted to make the rules resemble the books and the model in some way, that being said I agree with some existing ones and really dislike others.
Palladins for me are ok with a +4 save they don't have shields, are infantry and I think the 3 wounds better describe how they can take a punch before hammering someone, also they are supposed to be the hammer not the anvil. What I don't like is the points cost and battalions that make no sense right now, and their rend should be at least -2 base for all and 3 attacks (at least). This means they are slow, not durable and hurt like hell if they get close.
Another thing for stormcasts, I havent read yet a book where Stormcast turn tail and flee, and I think the battleshock system should be reworked. We should have a very high bravery IMO.
I like the To The Last Man standing rule but I would make it a command trait or battalion. Stormhosts vary a lot, some are super tactical some are more on the Zealot side and I would put this rule in one of those. It will also make choices more interesting depending on the army list you build.
About the Celestant Prime, I think he should start with a flat 5 attacks, thats 15 possible damage if you put him on the table turn 1. Bearer of the Warhammer: 14" Immune to battleshock bubble. If this model is slain all STC units in the bubble take a battleshock immediately. Fury of the Storm: For the first turn of a combat this model and all Order wholly in 9" reroll 1s to hit. Lastly a +5 w and mw save. It looks overpowered in a way but every time I read that he is weak I just remember the first book where he shows up and kills 3 GUOs in less than 5 minutes...
The main thing I see that GW did very well about Stormcast his how at the end of a battle almost every model is dead like in the books..