I also rather like the idea of of avatars, while being largely similar, having minor or even major differences in appearance and wargear reflecting the differences in the craftworlds' psyches and approaches to war. Altansar's Avatar (oh hey, that's got to look freaky after all that time in the warp!) might have a scythe or even a mega-maugetar. Iybraesil's wailing doom might be a triskele or an executioner, and the avatar might have a banshee mask debuff effect. Saim-Hann's might have wings or simply be super-fast and use the spear or lance for it's wailing doom representing the lance weapons favored by autarchs and shining spears. Iyanden's might draw a lot of its psychic fury from the grief of the dead and trade out part of its buff aura for some sort of soul-sucking or vengeance effect. Ulthwe's might have some blend of wailing doom and deific wychblade/singing spear or a special rule that represents the seers' certainty that the Avatar's use is necessary. Stuff like that.
I also really like the idea of the avatar having an extra minor ability reflecting the shrine to which the Young King belonged. Wings for a hawk, a flamer template attack for a dragon, etc.
...To the forge!