Okay, given that Yrael is supposed to transform into something new, I would suggest starting from a
Trumpet Archon, scaling it down by 1 or 2 HD to fit his current level and then change a few details to Imperial aesthetics over archon-like ones.
This works optimally for his Cleric-casting, as it makes him effectivly a full Cleric without the need for the template, it leaves some space for him to keep his Second Skin ability (no armor-penalties) instead of the Trumpet-ability and as magic weapons he can get something in Valyrian Steel to go with his grey wings, while keeping the total power of the weapon similar to the Trumpet's +4 Greatsword.
@Goldfish @DragonParadox
Do you think the concept is solid?
I see no reason to re-invent the wheel here, as even an imperial-aligned Archon is still mostly what he was.
We do not have the metaphysical backing yet to make completly new and independant outsiders, to change an Archon as much as Hell changed the Fallen, so I would much prefer to keep it to minor alterations.