So I was playing around with the Eidolon, and one of the things I tried to do was "recreate" an angel as an eidolon. Starting with the traits of the subtype, I tried to stat it out as similarly as possible.
Darkvision 60 ft. (free)
Immunity to acid, cold (4 points)
Resistance to electricity, fire (2 points)
Magic Circle against evil 3/day (4 points) and Lesser Globe of Invulnerability 3/day (5 points) to simulate Protective Aura
Tongues 3/day (4) to simulate Truespeech
So that's 19 points right there to just cover the basics of an angelic eidolon (not including other staples like winged flight, magic attacks, etc). I imagine that someone trying to put together a similar eidolon but of other outsider subtypes (archon, azata, demon, devil, elemental) would face the same problems.
My suggestion is include tiered subtype packages that summoners can apply to their eidolons that actual apply the subtype and can be paid out over the level of the caster (possibly at a discount, as a reward for staying within a theme). It has to be purchased at first level and cannot be swapped into or out of later.
Angelic subtype (2 points at first level, 1 point every odd level)
Prerequisite: Summoner must be Good.
1st level - Gain resistance to acid 5, cold 5, electricity 5, fire 5, low-light vision
3rd level - +2 to saves against poison and paralysis
5th level - Resistances increase to 10
7th level - Gain immunity to acid and cold, +4 to saves against poison
9th level - Gain truespeech (as tongues, at-will)
13th level - Immunity to paralysis
15th level - Gain angel subtype (including protective aura).
That works out to 11 points, which is a significant discount (so maybe would need to be adjusted upwards), but the eidolon would have a significant portion of its points committed to a theme.
this isnt a horrible idea. I think also, that the Eidolon should have to pick early on between a bestial and humanoid form. If he picks bestial, then he can get natural armor and natural weapons. If he picks humanoid, he can wear armor and use weapons. Or, at the very least, the choice could adjust the cost of the other side, to prevent the Natural armor, Size Ac, and Armor bonus cheese weve been seeing.
seems fair to me.