What should I pick? (Overpromotion problems)

Fade Cloak seems the obvious choice for Keeper. It's always been a sister skill to Fade Step for me. Otherwise, I'm thinking Veilstrike as well, with the Twisting Veil passive. Something like this. I'm assuming you don't really need to go far into the left tree, so you could probably make the most of a build like that to kill even faster. If you want to go full hipster, you could also replace Barrier with Mind Blast. Chain Lightning -> Veil Strike to put enemies to sleep, then Fade Step into the midst of it to Mind Blast them for the nightmare combo. They'd have to survive the Chain Lightning and possibly the Fade Step for that, though, and of course there'd be no serious barrier xp.

Katari is a harder question. It's either going to be Block and Slash or Whirlwind. Not a fan of Whirlwind myself, but I have to admit that using it around a Frost Flasked Luka would be interesting.

While I don't have the WIL that you do, Sassa, I do find a 4th skill redundant on Keeper. Building her with passives mostly then waste it all away to start back at level 1. At this point, I treat her as a very busy support mage. More fun trying to barrier teammates spread out all over zone 5 on FC 

If I ever find myself at 1000 on WIL, I will play Zither until another BW game comes along 

Well this build seems seriously fun. Although there could be a problem with Venatori faction since their mages are immune to sleep when they have barrier on and there are a lot of them...I agree that if go full dps fade cloak is probably the best 4th ability. With near 1k Willpower you'll probably blast those electric mages onto the moon.

Also for anyone who is bored of unable to be killed, I wholeheartedly recommend Body Guard on Templar. Finally got a chance to try it and I like it so far. (Special thanks to coldwetn0se for picking Avvar for this test  )

What I find interesting is that when your teammates are in the range of Body Guard, all the damage they take add to Templar's Guard if she has shield wall on.(As shown in the beginning of the video when the bolter shot Avvar from behind.) I got a decent amount of xp in the end because almost every foe hit my shield, either directly or indirectly. Thanks to Wintersbreath ( 50% damage bonus, 50% damage taken from all sources) I almost died once or twice even though I was level 19 with all the possible damage resistance passives added. But fighting amid a horde of bolters with so many red numbers popping up is quite fun. Haven't felt such kind of thrill for a while with Templar.  I'll probably put on Belt of Urgency on her next time, hopefully with a full group.(HB maybe?)