It's a decent list, and you've pulled out some excellent units for it, but... I'd make a few changes. First, though, I'm gonna reply to a few of your comments:
I chose Karanak over a juggernaught herald because he'll give that big squad of Bloodcrushers scout!
Karanak gives some spectacular anti-psychic capability and a whole bevy of useful special rules to units he joins, like Scout, +2 to DTW, his locus... it's a shame that his statline, with AP- and no rending or anything, is so unimpressive in combat. For fewer points, I can get a Herald of Slaanesh on a steed with Beguilement and a Greater Etherblade--hnnnnnnng. You're right to point out that he's better fit with non-Flesh Hound units, though honestly, I prefer to add him to Bloodletters, since Bloodcrushers have some damnably painful downsides: T4 and only a 5+ invuln, for one thing, so anything S8 or higher that gets leveled at them causes a whole bunch of Instant Death on your very, very expensive minis. I'd much rather take, say, 20 Bloodletters with a Banner of Blood--who still get to Scout 6" forward, helping them with their big lack-of-mobility flaw, and have more wounds (and more attacks on the assault) than the 6 Bloodcrushers who cost roughly 100 points more than them... and can't be IDed in 3-wound chunks.
And with that hundredish points, you can buy a squad of Fiends to assault in alongside them to A) drag the opponent's I so that you're striking before, rather than simultaneously with marines (or rather than after Eldar/BA, or at the same time if assaulting into cover), and B) soak up overwatch with your Fiends. And C), as a bonus, stack a little more anti-psychic 12" bubble in the area.
But if you're taking Bloodletters at all, you have weigh 20 Bloodletters and Karanak vs 20 Daemonettes and that Herald I mentioned... the Slaaneshi rending fest has a lot going for it. And has some points to spare for mastery levels on that Herald to shoot for another Invisibility! And can rend T8 Wraithknights or 2+ save Termis, Dreadknights, Riptides etc a lot better. And have more attacks. And have Fleet and +3" runs to get into combat faster. And...
Sorry. Karanak's anti-psychic capabilities are a very tempting thing to take. I keep trying to make it work (say, in a list where I also have Skarbrand DSing in on the Banner of Blood, so that he doesn't scatter and all my assault-on-T2-with-I5 Slaaneshi units get his aura of rage, while Karanak is helping guarantee that the Bloodletters can get that assault thanks to his Scouting)... but it's hard to justify, compared to what else you can get with his points.
For the Heralds of Slaanesh, I'm torn between the etherblade/swiftness and the beguilement locus, which one do you think will be more useful? I like the etherblade/swiftness because it pumps them all up to i10 and there's an ap2 weapon in there in case I can't get enough rends, but I feel like if I charge through cover thats mainly wasted, and I can get more rending hits (and hits in general) with the beguilement locus. I feel like the big squad of bloodcrushers with invisibility will just be so SO mean... Thoughts?
Beguilement absolutely wrecks units. Swiftness will let you strike before most Eldar (or the new Blood Angels), or opposing Slaaneshi Daemons--so long as you aren't assaulting into cover--but Beguilement is definitely worth the price hike. And... why fret the small stuff? Add in a Greater Etherblade, not one of those lesser things. The +1S on a unit with a bunch of attacks (rerolling to hit thanks to Beguilement, with AP2 and I7) is absolutely wonderful. I want that S5, not S4. Worth the points. And add extra movement and another attack in for a few more points with a Steed, if you can at all spare it--plus, then, you have the option of Outflanking the Daemonettes with Acute Senses to have them turn up from reserves on whichever side of the board you want without
having to wade through about two turns of enemy shooting across the board to get there.
*edit: one thing i'm pondering is how I pop transports at range with this codex. It seems like something I should include so I can charge guys inside, but I'm not seeing how I do that reliably without using a tzeentch herald + pink horrors for s6 spam. ideas?*
That's... honestly the looming problem that hurts this list most. Not just how they take out transports (and honestly, against rear armor 10 on most transports, the Flesh Hounds and Bloodcrushers with Furious Charge assaults, or Daemonettes with Rending, will be able to crack most transports that they can reach--and if they get a far enough assault, they can wrap around the transport's access points to force an enemy Emergency Disembarkation), but how they take on tanks in general. Be'lakor and 3 Screamers are not enough heavy mech list--say, with 2 Ironclad Dreads drop podding in (plus the AV 12 Drop Pods), then 3 Predators, and also a Storm Raven. Or, say, an enemy Daemons list that brings 3 Soul Grinders. Or wave serpent spam. Or any list with a lot of AV 12+ vehicles...
You're gonna need some more anti-tank, honestly. At least 3 more Screamers in that Screamer Squad, I'd say, to start out (and they stick close to Be'lakor, who casts Shrouding, so that they can Jink for a rerollable 2+ cover--heehee!). Probably also a Soul Grinder, honestly--he's durable and can dish out almost as much pain as he can take.
While I like the Plaguebearers in principle--Poisoned 4+ and Touch of Rust letting them threaten any non-flyer, and Shrouded buffing their defenses--they're sooo daaaaamn sloooow. And if they DS in, they can't run that turn (Slow and Purposeful!), so are helpless and tightly packed against templates and blast weapons. That in mind... they're basically absolutely fantastic if an enemy is coming at you full tilt, but when you're closing on most enemies with your own army, plaguebearers are just too damn slow to keep up. I would probably drop them and the Herald of Nurgle entirely, to be perfectly frank. Perhaps take 3 bases of Nurglings that can hide better and have more wounds than a Plaguebearer squad for half the points, if you want a cheap home objective holder (and still get Shrouded to hide in ruins).
I'd feel most comfortable with another few Flesh Hounds in each squad, so that your opponent can't focus fire one group of them for a relatively easy First Blood. And if you drop Karanak and his Bloodcrushers, you can afford a Juggerherald or two to turn those fast, cheap tarpitting Flesh Hound units into real pain-dealers thanks to AP2 etherblade wielders. Who have a few decent locuses to choose from!
Or you could fold both Flesh Hounds into one squad, hehe, with one Juggerherald--then bring in some Seekers, too, for more fast, fragile pain-bringers to dart up in front of the Daemonettes. Or to Outflank.
Be'lakor... is spectacular. And Spectacularly fragile, for so expensive a mini, so always remember to jink for that 2+ cover. As long as you manage to get off Invisibility and usually Shrouding, so... you might want a few more mastery levels in the list, if possible (on the Heralds of Slaanesh, perhaps?) to guarantee that you roll up enough dice to cast them reliably.
Some thoughts. Do with them what you will. More anti-tank is kinda necessary, still, even after my half-hearted suggestion of "more screamers and maybe a soul grinder." Allied CSM could provide some real help, there: a Maulerfiend and a Predator, or some Havocs and melta-Bikers, would go down like a treat!