I would look at running a pair of Deamon Engines, make them the same.. and given the rest of the list I would say a pair of Forgefiends may work the best.
I would also consider, 'mobing up' the Cultists into one blob.
You might face a pre-assembled bias there, honestly. And it seems like the cultist load-out is a pretty "Dark Vengeance" inspired one... that said, if I had the opportunity, I would still take a purely melee cultist unit of 20 over the 10 and 10 melee-and-ranged on offer in this list, even if that means proxying the 10 autogun cultists from Dark Vengeance as melee-pistol cultists among close friends.
Ahem. Based in a purely hypothetical stance, it might be best to run two of the same Daemon Engine, yes: redundancy means that even if an enemy takes out one of them, the second is still ready and kicking to fulfill its designated roll. That said, the flexibility offered by fielding a Maulerfiend and Forgefiend alike means that you are more capable of responding to varied threats at range and up close, so that has a value and capability to respond of its own... (assuming that your foe doesn't surgically remove whichever is more threatening to their particular interests, which is basically what a competent foe will do, given the opportunity).
So what I mean to say is: if you, moshpiler, have already built these two minis of the same kit in opposite ways, no major foul. If you could still build them the same way, so much the better and so much easier to build a list around. If, in best case, you could (say) magnetize the units so you could field them in either Mauler or Forge predominance, so much the better for your collection and list flexibility!
Myself, I try never to field a Burning Chariot unless I'm also bringing (the somewhat unreliable) Grimoire or (the dispellable/failable) Invisibility, as major defensive buffs. Nor am I too overly impressed by the AP4 Flamers, so I might kill 2 birds with 1 stone and drop the Flamers, take the Grimoire on the Herald, and still have a few points to play around with--say, grabbing another Screamer or two. If you don't want to bring the fragile-as-all-hell Burning Chariot, you can always resort to the durable-as-all-hell Soul Grinder, who for a nominal cost can still fit in with your Tzeentch theme--and be more survivable to boot!
I would also try to grab a Rhino or two for the 1ksons, more for the sheer number of Ob-Sec units I'd be fielding than anything else, but also to boost their mobility.
Giving you MLs, Daemon Engines, Rhinos and more Screamers to think about, I hardly think I have limited myself in terms of how to spend that hypothetical 66 points that are still free (even if you drop the Flamers to fill out the rest of those options), but in terms of potential long-term growth, I think that every single one of those option offers areas for expansion. I might, admittedly, default to Be'Lakor at the 1850 point level, since when he casts Shrouding, it means that nearby Screamers have a 2+ rerollable cover save, thanks to Jink and Daemon of Tzeentch, and he can always Jink for a (ha, "mere") 2+ cover save with his native Shrouded, and Armorbane+Fleshbane at I8 means he can take on/out most things in the game before they can so much as force him to rely on his 4+ invuln... but this is a definite personal bias speaking, so take that as you may.