1. Beastbonded Witch I suppose could add fast healer to a familiar - which means that every time it gets magical healing it would add a con bonus. Binder Wizards could also.
Additionally, I think spirit shamans can apply these feats.
This is interesting for:
Pooka's (Fast Healing 2) Con 13
Protean Voidworms FH 2
Tripasaura FH 2 Con 12
Ceru FH 2 Con 14 ***
Mephits
Nycar
Imp Fh 2
Additionally, a valet familiar could split this healing with his master via share healing.
Honorable mention: Scarify spell which would allow you to effectively double healing.
Spirits Gift (see below), especially life ability.
2. Additionally, Poison effects are increased to the DC of the polymorph spell. Certain familiars have the ability to cast polymorph or change shape, which would boost the DC of their save.
Humans Eye for talent additionally can increase the con of your familiar.
Notably:
Tripasaura: 1d2 Wis Save.
Nycar Cannot cast polymorph but poison requires 2 saves.
Imp
PseudoWyvern
Typilidae 1d4 con Dam, 2 saves...
3. Swarm Monger Druid have the ability to have a familiar which can become a swarm. Certain improved familiars have great AoE resistances or immunities making them great swarm candidates. These include:
Angel Cassisian: immune: acid, cold, petrify. Res 10: Fire, elect
Shikigami Res acid 10, elect 10, fire 10
Aeon, paracletus elect, fire 10 : good aura effect.
4. familiars with interesting crits
Notably
Brownies 19-20
Shikigami x3
Liminal Sprite Rapier 18-20
5. Wysps give significant advantage to summoning creatures of an elemental type. Strong advantage for summoners, ifreet, sorcerors etc.
Interesting feats:
Spell sponge: Double the duration of spells
Mauler's Endurance: Familiar gains +2 hp per level
School Familiar... Interesting for Evocation school and others
Improved Familiar Bond.
Evolved Familiar
Familiar Bond
Boon Companion
Special delivery - theoretically multiple casters could be used power the touch ability. Also certain familiar archetypes gain (valet?) gain whatever teamwork feats you have. Such as special delivery.
Improved share spells
Group Shared spells: Aka: monster familiar.
6. Spirits Gift:
Ignore difficult terrain, fly penalties
Slums: +4 on initiative
Stone: DR/5 Adamnatine* yes please!
Water: Mobility as bonus feat. Breathes underwater
Air: Light elect res 10
Ancestors: Familiar speaks bonus languages. Choosable every day.
Nature: +2 Nat armor bonus
Bones: Blur
Flame: Immune to fire. Vulnerable cold
Heavens: Star map, Fly speed.
Life: Fast healing 1 (See #1 above)
Lore: +2 init +4 Stealth
Mammoth +2 strength
Wood: Gains Freeze: Ability to take 20 to hit in plain sight **
Strong, due to versatility
7. Interesting spells
Familiar image; Mirror image for your familiar. But better.
Polymorph familiar: Way to boost poison dc's.
Shield Companion. Huge increase to your companions hp; minor boosts to ac and saves.
In (1) you're not trying to get Fast Healer to trigger on fast healing, are you? The former requires magical healing, the latter is always merely extraordinary AFAIK.
No,
I'm looking at the combinations of scarify and/or infernal healing, combined with the shared healing feat, and/or shied familiar spell.
A familiar with a 20 con (not too hard), with scarify if you cast infernal healing on it would heal 12 hp / rnd (6 real, 6 non lethal)
Seems... interesting, especially if you can heal at distance.