Always nice to see other Ksons players and not just those posers who only take Magnus.
Here are my thoughts:
Exalted Sorcerer with Seers Bane
I love Seers Bane--it's a real killer. I'm not overly convinced that paying out for the Exalted Sorcerer *and* Seers Bane is worth it. The primary reason to take either of them is if you intend to get in a lot of close combat with Gargantuan or Monstrous Creatures. Your list, at the moment, doesn't seem designed to do this, so I think you could save a lot of points by just downgrading this guy to a regular Lv3 Sorcerer with a Familiar and get more mileage out of him. Either that or tool up those Scarab Occults to wreck face in CC and stick this dude on a Disc (the extra attack would be clutch, too) and put the hurt on something.
Sorcerer with Mark of Tzeentch, Aura of Dark Glory), +1 Mastery Levels, Veterans of the Long War, Force Axe, Astral Grimoire, Jump Pack
I like this and I like the idea of Jump Terminators. Do it.
Scarab Occult Sorcerer
4 Scarab Occult terminators with 1 Hellfyre missile launcher
I like this, too. The Hellfyre is the only weapon upgrade that really seems worth it. The Soulreaper Cannon is garbage.
There *is* a case to be made that a squad that has the Astral Grimoire can actually get some mileage out of the Heavy Warpflamer, as they're fast enough to position it. Worth considering.
Aspiring Sorc and 7 Thousand Sons with 1 Warpflamer in rhino transport
Aspiring Sorc and 8 Thousand Sons with 1 Warpflamer in rhino transport
I am not sold on the Warpflamers in Rubric squads, myself. S&P means you can't really get the most out of them and they just make an expensive squad even more so. I guess maybe you can drive around and flame people, but still...
Hellbrute with Multimelta and Power Fist
Predator with autocannon and lascannon sponsons
I like all this. The Defiler never lets me down and Hellbrutes and AC/LC Preds are cheap and fairly effective.
Jormungandr (Lord of Change) with 3 Mastery Lv
Decent choice, adds some psychic power, etc.
18 Pink Horrors
18 Pink Horrors is a little suboptimal in my view. Downgrade to Blue Horrors and take three units of 11. That doubles their psychic output and, even if you lose a Blue Horror, you immediately Split off a Brimstone Horror and don't lose any warp charge. It even saves you a bunch of points.
By my reckoning, this list is throwing down 14 Warp Charge, which is okay. With the alterations to the Pink Horror unit and the Exalted Sorcerer, you could easily be throwing down 18 Warp Charge, which is better. The primary drawback here is any reliable kind of mundane antitank. Yeah, yeah, you can pray that you can get enough decent Heretech in a list to stop an armored assault, but I don't find that precisely reliable, myself.
I think maybe the best improvement would be to buy a disc for the Exalted Sorcerer and put him with the Termies and the Jump-Sorc and have yourself a nasty little super-unit. To do that, you'd have to drop both Warpflamers, switch to Blue Horrors and then scare up another 5 points or so. That would give you a solid melee unit to back up your stalwart objective holders and shooting units. You're going to be wildly outnumbered at 2000 points by just about everybody, so having an assault phase is going to be key.
Good luck with it!