PoB: https://pastebin.com/3qXd3JCF

Theorycrafted this as a potential second or third build for this league. There are a collection of core concepts that I'm fusing together:

1. Indigon with a lot of mana spent is a large damage increase, but hard to sustain.

2. If I'm stacking mana for Indigon, it needs to also be doing something else to be valuable

3. Eldritch Battery provides a lot of effective mana regen in the form of energy shield recharge.

4. Arcane cloak spends mana, but does not cost mana, meaning it affects indigon in spite of Eldritch Battery.

5. If I'm relying on Eldritch Battery for mana costs, I need to not have it take damage.

So, let's get fusing. The goal is an Arcane Cloak with a combined cooldown and duration of 4 seconds to regularly refresh indigon's % increased spell damage. Stacked mana plus Mind Over Matter gives us a large pool of hit points. Heirophant lets us get that mana as extra energy shield, and with 50% less cost of skills helps deal with the massive cost increase from Indigon. Then, take Divine Flesh so that your energy shield never takes damage, so its recharge is always in effect. And, Divine Flesh helps mitigate elemental damage by converting it to chaos, so why not use Annihilating light? A few sources of mana recovery on skill use should endure that we're at max mana every time we want to use Arcane Cloak for max damage. And, since Arcane Cloak spends mana through Eldritch Battery, we can use sigil of power for extra added lightning damage.

Finally, we're mana stacking, so we may as well use Lightning Conduit to make use of the increased mana applies to shock effect. Conveniently, Lightning Conduit and Storm Brand have the same cast speed / activation frequency, which should allow us to cast a brand and then spam cast conduit. And if you don't want to clear with stormbrand (like me), I have it in a 5l setup in the staff that you can add whatever clearing skill you want, like arc.

Notes: I have 65% shock in the PoB even though I technically don't reach that with current setup. I figure a bit better gear and it's reasonably achievable, so I left it. Defenses other than an 8k health pool aren't great, but may be enough. Recovery in scary situations is a bit flask reliant, but with indigon's health from mana flasks and MoM plus leech I expect it will be okay. A rare amulet with lightning damage leeched as life plus other mods may be more useful than Atziri's Foible, allowing you to take the charge ascendency node instead of leech, but I was too lazy to investigate.

Edit: as pointed out in a comment, I was mistaken about energy shield recharge: any damage to the character will stop recharge, not just damage to energy shield. Enough faster start of energy shield recharge may make it still usable, but that hurts the build significantly.