Or perhaps more generally, a caster who has no intention of using weapons.
It seems to me that metamagic feats are generally poor picks, since higher level spells tend to be superior to the lower level spells with a metamagic on it. And there are rods to do it.
Item creation feats need a campaign with the downtime to use them, and essentially you're just selling your feat slot for tasty discounts on a type of magic item. This doesn't seem too appealing either.
So what's left? Take Spell Focus if you mostly keep to a single school? Spell Penetration if you're getting high enough level to meet lots of SR creatures? At least Summoners have a sort-of tree of feats they like, in Spell Focus -> Augment + Superior.
I feel like when I'm making a meleer, I've got many more interesting candidates for feats, but when I'm making a caster, I'm just taking Extra Class Feature nearly every time. (With a break for Improved Familiar, mostly for fluff).
Going back to the matter of witches, all I'm really seeing I should pick up is Accursed Hex, for those smaller fights. The fact that Hexes are supernatural, thus can be used while threatened with no problem, and don't care about SR means I don't care much about feats to bypass those problems either. I expect that if I hit level 20, my witch will have Accursed Hex, Split Hex, Improved Familiar, and the rest is all Extra Hex.
So, are the options really lacking as much as I think they are? Are there cool feats out there for pure casters (or specifically witches) that I've missed?