Why the ultamarines? They're crap.
Any way if I had to be honest I'd rather not have one codex to rule them all, I just think it would be a nightmare, especially when BA, SW and DA players want their specific dex's and not a yellow pages where they only use a third of it, yes you probably could fit all the characters, vehicles and units into a book about the size of the nillarines dex, but don't forget most 40K books (including BRB) are about 30% fluff.
I'm probably gonna get flamed to death for this but I would like to see MORE SM dex's(yes you read that correctly) salamanders and iron hands especially
When evil seeks to overcome good
And firepower ovecomes the spoken word
Then death shall rain from the sky on wings of blood
And retribution will be swift