My intended plan is to trade unwanted T5-7 essences for the ones I do want.
T1-4 essences work like an alch, T5 I am unsure, T6/7 work like an alch or chaos as appropriate.
What I'm talking about is the idea of alt-spamming a desired non-essence-capable top-tier mod on an item, let's say a prefix, regalling, then crafting "prefixes cannot be changed" from a level 8 master, then using the essence. "Prefix/Suffixes cannot be changed" do not prevent the addition of more mods of their category, they only prevent the existing ones being changed (has been tested/confirmed with both exalts and chaos already).
This will allow, for example, crafting a Flaring weapon and essencing Merciless onto it, etc, at the cost of a couple exalts for the metamod.
Or, in my case, buying 6L Vaal Regalias for a couple exalts, then getting crazy ES on them. In prior leagues a 6L with 800+ ES cost at least 20 exalts, even if it had no other desireable mods. In this league, you can make one yourself for ~8ex.
THAT is why I'm excited about Essence. I can make 8ex in a week. 20ex was nearly 3 weeks of play, at which point I burned out on a character.