tl,dr: Reaper Form is too temporary, and does things the Mastery Tree should do - and the Mastery Tree does things Reaper Form should do. Alternatively, use Reaper Form for a build-enabling skill, rather than just a buff.
Personally, the things that need improved for Reaper Form that it feels like a long-cooldown transient buff, not a true Transform. The extra hitpoints we’re granted are drained away continuously, increasingly fast, and specifically excepted from the other skills we get that may circumvent this. The nodes on the tree are just “do more of what you do otherwise, while transformed!”, there’s no benefit to having the skill and not being transformed (not that I think any of the transforms in the game have that) and so we’re forced to wait a long time for it to cooldown, but also nothing unique about being transformed, that shouldn’t just be on the Mastery Tree.
I think that all mastery-trees should have nodes specifically for the mastery-skill they provide, to incentivize using those skills - similar to Druid’s Rage Decay reductions (and specifically looking at Rage does not Decay outside Combat), Beastmaster’s Aspects gained by Companions, or Spellblade’s bonuses with Elemental Attacks - and Lich has to put in extra work to make that happen with Reaper Form. The specifics are always going to be “wrong” according to somebody - but the general form needs to synergize. Having bonuses While Transformed on the Lich mastery tree would go a long way towards making Reaper Form feel like THE skill it’s supposed to be - not just a transient buff you can include in most other Acolyte builds. The Mastery-tree nodes need to support the style - the theme between the skills themselves - not just the mechanical way we apply damage to monsters (though plenty of that is appreciated, too…)
Since most of these nodes are already on the Reaper Form skill, it would make sense to swap out some of the nodes from our Mastery Tree to the Skill tree - Increased Damage on Kill, Increased Leech on Hit, Leech Converted to Damage, Leech Converted to Ward (or per Second), Reduced Health Drain/Reduced Health Cost for Skills, or Low-Life skill-nodes are some ideas.
I think that providing another avenue to enable low-life for Lich builds is important, and Reaper Form tries to do that (especially since Death Seal prevents you from generating ward, like most items that drain your life provide you). Reaper Form could be adjusted from continuously increasing decay to be a flat amount of “x% of current life drained per second” - or a multiplier for effects that our Lich may already have doing that - then the theme of “Lich’s Increase Damage at Low-Life” from our Mastery also fits with the Mastery-Skill, and not just temporarily - until we de-transform or leech ourselves back up trying to sustain Reaper Form. Right now, Reaper Form mostly feels like a defensive layer while using Death Seal, and while that one-two punch is fun, it feels like a weird love-triangle of “you want low-life, but you want all the leech, and you need more hitpoints so being on low-life isn’t actually dangerous because you can’t have enough other defense to make low-life worth it while Death Seal is on”
Take for illustration the numerical balancing we would have to do to make Endurance our primary defense layer while low-life, with no ward: We’d need to balance our Low-Life threshold number at our Endurance Threshold number, max our Endurance percentage, and then scale this whole system up with as many hitpoints as we can cram in - so that some of these 1200-1400 damage (after resistance) single-hits don’t immediately break our Reaper Form or one-shot us - and add to this that getting “increased Endurance Threshold” or “More Endurance Threshold” anywhere on gear is much harder than getting the comparable Life mods.
Probably ended up raving there at the end, but I appreciate everyone who stuck with me through that - happy hunting, Travellers!