I have been playing around with the following build idea since the level 20 gem info was released: Scion (Gladiator / Pathfinder) Reap/Exsanguinate (going to have to test which one ends up working better and how Reap overlaps work with Spell Cascade).

Now to preface, this build is based on the assumption that Indigon is not changed and that spell Life cost is not going to be increased by it, so we can proc it using Arcane Cloak / something linked with Archmage and get the bonus for our life-cast spells.

We grab some block (the tree and items are pretty barebones, so I'm reasonably sure that with a bit of farm we can get a decent amount more block, life and mana, these are just pretty bad-case-scenario options). We abuse mana flask sustain via pathfinder and mana flask healing via Indigon to have a constant inflow of Life to both sustain damage taken and spell life cost. The preliminary PoB is here: https://pastebin.com/89AJ27U4

Anyone else working on something similar?