Edit: The more I think of it, especially compared to what 1k Sons sacrifice in order to take their Grand Coven, the more I think that their detachment bonus should have been "+1 Mastery Level" for all their characters, rather than "an additional spell." If I face down a Grand Coven, I would be cool with my opponent using that houserule. Currently I'm working on updating my Traitors Hate army for Word Bearers.
Yes yes yes. The current Grand Coven rules are pretty lame. Re-rolling Perils is nice, but you're still suffering Perils and few of those choices are "good." The extra spell to cast thing is pointless, since you will pretty much never be in a position where you will run out of spells to cast *before* you run out of warp dice. Well, unless everybody's locked in combat, but then you're more or less screwed anyway.
I think the only house rule I'd push for (and I may just) is removing the stupid requirement to take a spell from the Tzeentch Lore while *also forbidding* you to take more than half your spells from said lore. It's basically a rule saying "no psychic focus for any of you losers!" That sucks big-time.
RelicsI haven't looked at the book in a day or two now, so my memory is slightly fuzzy, but here's my thoughts on the new Ksons Relics.
Seer's BaneThis is a pretty sweet piece of kit. You're knocking down almost anybody with this thing, though you need to pay a pretty penny for it. Ld10 doesn't quite grow on trees anymore, so slicing through Greater Daemons, for instance, just got a hell of a lot easier. You could make relatively short work of a Wraithknight with this thing. I like it. Of course, it costs in an army that has very few points to spare, so I can't say I'd be fielding it often.
Athanaean ScrollsThis has been getting all the press and I can see the appeal, but it seems awfully fiddly to me. Consider this: in order for it to work, you need some kind of doubles to appear on the dice. In order for this to actually be feasible, that means this is probably a power where you're rolling 4+ dice. In those instances, you are very frequently rolling sufficient successes that your opponent won't even bother trying to Deny, since the odds of success are so low. Instead they're just going to save their dice to deny one or two of your low-wattage witchfire attempts, at which they will be modestly successful. The exception of this is using it as a way to get around lots of enemy psychics boosting Deny rolls for their units. So, you know, it's *okay,* but not an auto-take.
Astral GrimoireThe idea of Jump Scarab Occult Terminators appeals to me. It appeals to me a lot. This is a nice, fluffy way to add mobility to a sadly very static army. You've got to shell out for a pretty expensive IC to get it, but you have to do that all the time anyway in this army.
Helm of the Third EyeThis is pretty okay. It becomes down right special if, for some crazy reason, you're putting Warpflamers in your Rubric units. That would be the optimal build, in my opinion, but, again, the overall cost of everything makes that an expensive and doubtfully useful proposition. If you're taking one giant unit of Rubrics with 3-4 Warpflamers in them, then yeah, I guess this would be boss. I don't see it as super useful elsewhere.
Staff or Arcane CompulsionThis is actually pretty sweet. It's cheap and adds a very handy bonus to the unit its in. Of all the relics, this would probably be the one I end up taking most often, given that it's affordable and a nice way to make assaulting my rubrics more inconvenient.
CoruscatorYeah, this is so-so. I'm really not crazy about small blast templates unless you can drop them in groups of 3+, so I'm doubtful I'd bother with this. This, like the Helm and the Scrolls, would fall into the category of "if I had points laying around." This is the Thousand Sons we're talking about, though, so that's never going to happen.