I would think of angels as very low growth, very powerful ... very good at diplomacy but can get angered really easily too.
Some (like the Seraphim) with heavenly fire swords, and great aptitude for fire spells (with which to burninate the enemy).
Others, perhaps Cherubim, could have access to spells of Ice and Life.
If an army of "all angels" has 2 or more moves, then they can spend one of their moves to "fly" over a blocking mountain or narrow sea, as long as its only 1 tile wide of course. (or N-1 tiles wide, where N is movement points).
However, angels will not fight alone. The majority of their forces will be converts and zealots (perhaps some slaves as well).
Their "Angel of Penance" (Or Angel of Repentance) could be a slave driver type, with access to the "Slave Drive" spell of the Magnar.
Their "Herald" meanwhile, could have a similar spell for those devoted to the cause (but better than Slave Drive of course)
They could have some life-based, or fire-based, spells such as "gods wrath" or "retribution". Probably not by an angel, but by a human priest.
angels should probably be immune to both life and death spells.
an Angelic faction (a mix of angels and their human followers) would likely have the least amount of unrest, able to squeeze high taxes at little-to-no cost ... allowing even their cities of limited population to be rather productive.
In the unit design, can choose either male, female, or Angel. Angels only have 1 trait slot instead of 3, and buy traits from a different tree. Angel Traits.
Angel Traits are powerful and unique. If you want a generic angel you don't pick one, but if you want a Seraph, Cherubim, Nephilim, Herald, Repentant Angel, Throne, Virtue, etc .... you would pick the appropriate angel trait.
Angels of "Mercury" or "Swift Wings" could be the ones with extra flight-based abilities you mentioned such as Thunder-strike or Sky-dive.
Virtues could have healing magic and buffs, while Thrones could specialize in Charm/Fear/Domination magic.
Nephilim could be the 'life stealers' that can cast fear and can also Drain life with ease.
(again, angels would be immune to healing magic(life based), so the Nephilim would be the only angels capable of healing during battle).
Air Elementals would be a good starting point for at least one angel type. (yes, angel traits could perhaps influence the unit art 
)
Angels without traits would be the simplest art/ look the most human ... while Thrones (and Angels of Mercury) would likely look the least human.
anyways, those are my Angel-based 2 cents for the moment. 
