I’m going to start with a few feats made for spellcasters that I’ve started to toy around with. Unfortunately, I don’t have a good environment to actually test these in, so if you have any input, feel free to comment.
Bodyguard summon (Metamagic)
Prerequisites: Augment Summoning
A Conjuration(summoning) or Conjuration(calling) spell modified with
this spell allows the caster or summon(s) of this spell to aid another
to improve AC as an immediate action. This aid another can only
affect the caster or a summon from this metamagic. They can also use
this aid another to give +1 to saves. Also, the caster as a swift or
immediate action can spend any amount of hit points to heal all
summons from one casting of this spell the same amount. This metamagic
increases the spell level by 1.
Special: This can by DM discretion, be extended to evocation spells
like Flaming Sphere, Grasping Hand, and Spiritual Weapon. If this is
done, then the Prerequisites change to Augment Summoning or Greater
Force Caster (Wizard)
Prerequisites: Spell Focus(Evocation) or any Force based reserve feat.
Any force effect that you create has increased hardness equal to your
casting stat and has it’s duration increased as if your Caster level is
2 levels higher. You gain no other benefits from this caster level
increase. You can as a swift action spend hit points to heal the force
effect for 3 times the hit points spent. Finally, when a force effect
reaches the end of it’s duration, instead of fading, it loses hit
points equal to twice it’s spell level every round. This effect only
applies to force spells that aren’t instantaneous, and have hit points.
Companion Spell (Metamagic)
A personal or single-target spell targeting yourself can affect a
second target within 10 feet of you. If you and the target are more
than 10 ft apart for any reason during the duration of the spell, the
spell immeadiately ends for both people. This metamagic does not
increase the spell level if used on an arcane spell. A divine spell
with this metamagic is increased by 1 level.
This last one’s just a bit weird, having different levels for divine/arcane. The idea is that arcane and divine casters would take different risks to be able to stay within 10 feet. On top of that, divine spells have some pretty powerful self-buffs.