I just found a small boost. A small one, but a nice one.
Fey Power, which requires Fey Heritage (+3 to Will saves vs. the entire Enchantment school), gives a +1 to the save DCs and caster level of all Warlock invocations (as well as Enchantment spells.)
Not super-duper amazing, but it is pretty damn decent for a feat.
EDIT: Another fun feat is Fell Conspiracy: You just need 4 ranks in Knowledge (Religion) and Wisdom 13+ to take it, and it lets you give the entire party Message at will for 24 hours as a 20 minute ritual.
However, since it's a [Ceremony] feat, it has 2 other benefits: if you have 8 ranks in Knowledge (Religion), you can use up 50gp to grab everyone 100' telepathic communication with each-other for 24 hours. And, if you have 12 ranks, you can spend an additional 150gp to essentially give everyone in that telepathic network the Hivemind special ability.
Ceremony feats are lovely for anyone who can afford the money burn; 200gp for every day of adventuring might be expensive, but the benefits are worth it.