I like the idea of storm shield Celestians without any form of tactical dreadnought armor, that just kills the idea for me. Squad has same stats and squad size of the current Celestians. Beyond their grenades and pistols, give them storm shields and chainswords. Chainsword can be exchanged for a weapon from the melee weapon or ranged weapon lists.
Give the unit a chance to generate an Act of Faith on a 3+ each turn that can only be used on themselves.
Currently the models are an easy conversion from a Retributor model, could be equipped with special weapons to be shooty or given melee weapons to be choppy. The self contained AoF would give them a lot of versatility (outside of a transport) and really allow you to spec them out how you want and allow them to be good at it. Properly costed, they would be a great unit.
Give us that and then they just need to fix the cost of the hand flamer. If GW
wants it to be half the hits and weaker than a flamer they shouldn't be almost the same price. A pair of hand flamers for 4-6 points for Seraphim might make them worth the points. Leave them at D3 hits with an additional point of strength and they still wouldn't be worth what they cost now.