New Dark Eldar

They definitely need more low ap weapons. And dildos.

Huskblade and various (current) Heamy wargear will become Relics. Fun fact, back in early 3rd we had an Arcane Wargear section before everyone else. Yeah, we're pretty much the master race.
Here's hoping that Succubi get a few more weapon options. As solid as the poison blade is for a super-cheap wound-bot HQ, some variety would not be remiss.

Drugs may see a slight alteration. They're really very similar to what we had from the start. I would be very surprised to see Night Shields change (after 2 books and 4 editions, it becomes a standard).
I think if the Klaivex could purchase a PGL, then people would start taking them (and running Incubi as just Incubi). I would love to see Tormentor Helms going back to doing something beyond the assumed stat boost (ie, grenade effect, etc).

I would love to see our flyers get Vector Dancer. We're Eldar, yo. Also, I wonder if the Void Raven will go away completely as it still has no kit. I give it 50/50 at this point, either it gets released, or it gets dropped. GW has made that the standard. Razorwing could pretty easily fall into Fast Attack as well, which would also open things up for my Force Org.

Mandrakes... need some serious luvin'. They suffer from the same problems as any other walking assault unit that doesn't have a 3++. At least they have a 5++ and S4... they have the potential to be a *lot* of fun... but they need a better starting point. I love their baleflame mechanic, but getting that first token is too hard without relying on a synergy system, which isn't how they're supposed to work.

Soul Trap I think will open up a bit. I'd love to see something like "kill a character, gain +1S. Kill an IC, gain +1S and regain a wound lost earlier in the battle". In challenges, of course, because that's what 40k is about.

I'm not (yet) sold that we'll get the same rules as Eldar. Mind, I would splooge if we got battle focus. It'd be sick. Foot DE may actually be a thing at that point. Except that they wouldn't.
Which brings me to vehicles... I honestly expected Raiders to go down in cost with the last codex. Rhinos went from 50-odd to like 35 or something, I think Chimera's saw some small drops... granted, we're toting proper AT weapons as our base gun, but still, Raiders were about the only option to stay the same cost (or even go up 5 points maybe? Can't remember).

Some other gun options would be nice. Unless you count allies, which is definitely a thing, we really lack when it comes to dakka options. We have poison, we have lances. Dissies are a thing, and scourges/reavers actually have a decent variety... but beyond that, we have nothing in the S6-7 range. Awkward.

Speaking of weapons, I would like to see Dark Matter weapons go down 5 points across the board. Blasters on par with the cost of a Melta, Lances with a Missile launcher (especially if these stay our only options). Shredders go up 5 points (to 10, matching Blasters) and gain Monofilament (this I can't see not happening, they're fluff'd as monofilament weapons). Maybe go up to 18" too... this way it's 10 point seither for the anti-tank, or the anti-infantry, and both are solid options. Go figure.

That's it for now, kiddos.

Shredders as template weapons would be great, but I don't think that'd go hand-in-hand with monofilament, so I think its one or the other. Personally, I'd prefer the template option, to give our warriors a little added Umph in overwatch. It would also give 20 man blob squads, with 4 shredders inside, some much needed staying power on backfield objectives.

I hope the duke and baron stay, just because they are so fun to play with and incredibly fluffy - but they'd need models, so I won't be surprised if they go the way of the dodo (ie, killed for their tender delicious breast meats). Their only saving grace is that there aren't any 3rd party options available, and kitbashing them from current DE kits is SO easy to do. Even just changing the instruction booklet for the hellions box to add a "to build the baron use these pieces" page would be a fix.

In my head I keep applying the same rules-relations between High/Dark/Wood elves to Eldar/Dark eldar, since they are the most directly-linked armies across both game systems. In wych case (pun) wyches getting rending wouldn't be amiss. Having the option for haywire grenades still makes sense though, I don't know why they wouldn't keep it. And since they have models for wych weapons, those won't be going anywhere.

I think overall they have (another) opportunity to take an army that thrived on 5th editions combat system, and bring it up to speed for 6th/7th to once again thrive in combat. They didn't take the measures needed to help the Nids, the other combat-effective army in the game, but I'm hopeful that the DE get the love to make pure combat armies a viable option in the age of overwatch. If Phil Kelly is at all involved, this might be a thing. He fucking loves dark eldar.

Yeah you're absolutely right - poison would be a thing, especially since its LITERALLY on everything that isn't a death-laser.