For a beginner caster, I’d not go for a lightning build. It’s good at early game and leveling, but pain in the butt to get gear for once you hit the budget ceiling, and has been nerfed several times (on all levels).
On the other hand, you can go for dark one (not very hc friendly to get due to the rocks in the area), or Morgoneth’s for cold because those pieces are also target farmable.
Right now however I’d pick a fire based skater sentinel. A bit like how in 1.1.3.0 @Udars adapted the then-current templar skater into his sentinel, I suggest adapting the aetherfire templar
Why sentinel? Two things: Bloody Pox and Curse of Frailty. Those are proc machines. You also get RR on your end of the deal from vulnerability, and the usual suite of heal + resist from Blood of Dreeg. You also get a few juicy damage conversion options rather easily by the time you get to the point where you might want it.
For aether to fire, there is the Chains of Ordas belt available in port valbury until you get your set and the blueprint for mythical crimson lotus. Despite not having the relevant +skills (It’s a sorc/defiler belt), the conversion will help out with aetherfire damage. Port Valbury is coincidentally where you can farm the Vanquisher Helm, which reduces the cooldown of Vire’s Might by 0.2 seconds, and gives +1 to all skills in oathkeeper.
You can also drop single target nukes with Doombolt, using the bloodsworn sigil medal and Blazeheart (guaranteed reward) to get all the damage conversions in place, with an incendiary shoulderpads. I imagine it’ll best be dropped in favor of sigil of consumption when you get a mythical grim fate, but until then its a flavor option.