Berserker can't upgrade to spirit line, can only upgrade to blood frenzy.

 

I guess you're proposing blood line for mage slayer in order to keep frenzy up to compensate the debuff duration.

There's not such debuff on mage slayer since the release.

I agree mage slayer should take blood line, mainly if dots can proc interrupt on a spell disruption debuff, but need to test.

Well someone already mentioned the UI bug for Berserker frenzy upgrades so I'll just bring up the 2nd point: Mage Slayer states it reduces all effect durations, both negative AND positive. Does this not affect frenzy? I remember watching a video that said it did, but this may be the typical "I trusted the character creation UI, and it was wrong."

Also why is the Barbarian's replenish-able rage on kill both a 50/50 chance and gated behind power level eight while the paladin gets automatic zeal on kill at power level seven?

Probably because the barbarian is generally better at getting last hits with their splash damage.

I would have liked it more if Corpse Eaters would just have consumed essence from nearby corpses passively "on the fly" and gaining resources back (at a more modest pace of course). Then you wouldn't have to fiddle around with Flesh Communion.

That's how Druid's spell that turns corpses to healing works I believe. That would have been quite cool.

Yes, there's also an item (helmet) that passively reduces the damage you receive when you are near corpses.

 

So - the mechanics are there...