Feylocks have a few problems: A binary spell list (enchantments either work, or they don't), a feeling as if they're the forgotten half of the warlock class, and as I've said elsewhere, a gap between 'fun' and 'strong' so wide that despite being pretty good, they're mind-numbingly boring to play.
So, these aren't really power suggestions, although it might tweak their power here and there.
Feylock as the forgotten half
Fiendlocks get what you might call a pretty sweet kit. I'm not proposing to copy them. I am proposing feylock spin on that inspiration, since it clearly works. Hear me out:
Shift Summon Shadow to a lower level, but put it a little closer to the old project image- a glowing, hard-to-kill combat clone that lets feylock take a little bit of the pressure off of themselves. No cripstrike, but maybe it has some kind of taunt that pulls NPC attention.
Fiendlocks get see invis, countering, frankly, one of the only defenses in the feylocks kit. Give feylocks access to dismissal, even it out a little.
The last thing I want to copy from fiendlock is blast effects, which brings us to:
Fixing The Binary or: relying on enchantment dcs on this server encourages mono-building.
Your DCs on feylock don't matter. Most at level content has highly inflated DCs, or are straight up immune like undead, constructs, etc. So, you build con with just enough cha to make divine shield work. You F1+M1 flare from level 1 till you get mind fog, and then you F1+M1 mind fog till 30, and beyond, for basically every single fight you're going to be in for the entire character's existence. Maybe a dispel here and there. Meanwhile, the fiendlock can dance between elemental types to buff his fiend, debuff enemies, vamp heal... yes, he's still F1+M1-ing, but there's some strategy to it. Some extra button presses, some summon management. Ice storm.
So. It would require a lot to fix all that, so I propose an old solution, that neatly splits the fiend/fey warlock types between summoner-single-target-debuffer with a big nuke spell, and wide-ranging-tricksome-love-spreader (where love is negative effects) with some illusiveness:
Bring back blast types for feylocks, with minor eldritch essence effects for flare only. A feylock casting flare can select between, as they level:
a chain blast, with diminishing returns per shot
hideous blow, with (weaker) spellsword-style mechanics
eldritch doom (a big AoE that targets reflex)
Along with this, some per-element effects: Cold with a slight slow, acid a DoT effect, Pos a very minor heal for allies caught in the blast (warlock excluded), magic does magic. A potential for your individual blasts to matter more (chains, saves, etc) means you would want a bigger investment in cha and dex to land them, and make those effects stick, which means you've got a choice other than the monobuild.
Edit: swap shadow shield for an epic spell of the warlock's choice.Other notes, since I already forgot my header titles:
Some spell swaps might be a nice choice, genuinely. The over-reliance on enchantment really doesn't do them a lot of favors, and with new hak spells I think there's room for the spell list to have another look at their list in general. Eyebite, nightmare, sound burst, dirge, and irresistible dance, at least, could go without being felt whatsoever, and shadow shield does not match up as a capstone to a 30-CL-buffed fiend+ice storm.
Some stuff I'd maybe suggest as swaps: Dismissal, disrupting hand, the new shadow x spells, wall of ice.
