I've played all Tzeentch for quite awhile.
I have to say, banking on Flickering Fire to do work is a losing proposition. you fail 1/12 casts, and your opponent denies at least 1/6 of those that do work, which gives you about a 30% failure rate right off the top. When you really need those shots to kill or soften something, it will fail (Murphy's Law applies on the table!) and you'll regret it!
Horrors can be quite survivable against things that don't ignore cover simply by going to ground. In Area or in Ruins that should give you a 3+ cover, and you reroll 1s...in the area of Terminator-level survivability.
Flamers...I don't know, I want to like them but the're pretty squishy, and the FAQ that changed how range affects how wounds can be allocated really reduces their usefulness. I'd give them a pass.
Screamers are still pretty good. Fast, pretty tough, able to engage any target...but make no mistake they're not particularly killy. A common archetype is the "screamerstar", where you pack in some Heralds of Tzeentch on Discs, roll on Divination as many times as you can trying to get Forewarning (4+ Invuln), then using the Grimoire on the unit...now you have the 2+ with a reroll that will make your opponents cranky. You're likely to have some Prescience too, which greatly enhances their killiness, and if luck is with you some of the heralds can take Flickering Fire giving you some unreliable utility shooting.
To make that work reliably though really requires Fateweaver. I tell ya, I hated Fateweaver when the book first dropped. To me he was 300 points for a reroll. After playing him for awhile now, though, I'd not want to try a Daemon army without him. He does provide the reroll (once per turn...that means on your turn and then on your opponents), he keeps you reasonably safe from the Warpstorm, etc. Moreover, he'll have some pretty decent shooting attacks too. Flickering Fire, Pyromancy (the Primaris is a great deckchair cooker!), maybe Shriek from Telepathy...or maybe some of the really good stuff, Puppet Master, Hallucinate, Invisibility, Iron Arm...he can be a lot of fun. Keeping him alive can be problematic, but with some flying-off-the-table abuse, and hiding for the early turns until the Screamers take out the things that threaten him, he can be a great late-game objective clearer!
Lords of Change are also very potent. T6 makes them pretty tough and immune to most Instant Death. A couple Greaters for some increase survivability (3+ armor, 4+ FNP, reroll invulns, an extra wound and IWND), a Lesser for a Staff for S8, and lots of divination for tricks. Not bad at all!
And my favorite, Daemon Princes of Tzeentch! With 3+ save they're near terminator save...plus flying. Lvl 3 for Telepathy (for the game-winning pwoers there) or Biomancy (damned good powers there too!), or even a Flickering Fire for some standoff ability! Add a couple greater rewards for some survival enhancing and maybe even a lesser for some S8 melee power, and you've got a pretty durable model that's extremely versatile and VERY hard to kill (using his maneuverability, ability to fly off the table, hide behind LOS blocking terrain, etc).
I personally think that of all the Powers, Tzeentch makes the best all-around army if you want to go all in.
My personal 2k army of choice is FW, Three Heralds LVL 3 on Discs (2 have exalted rewards for Grimoire and Portal, other has Exalted Locus for S6 FF), 3 units of Troops (I mix in daemonettes now, but all Horrors works fine) and two full loaded Tz Daemon Princes (355 pts each!! Be conservative with these guys and they'll win you games!)
Good Luck! Let us know how your army evolves and how your games go!