Fateweaver is a solid
HQ choice for a tzeentch list, is very fluffy, and makes your 4+ saves go a long way. What you really should think about is finding a way to get icon's in your horror squads, to aid in the rest of your army appearing. Also, dropping them to 9 models a piece is fluffy because 9 is tzeentch's number.
The demon princes make very good "snipers" because of their decent
BS and 4+ save, but without wings they aren't very mobile, so I wouldn't take breath on the second one. Chances are he won't be close enough to use it that often. Just stick with the longer ranged shooting powers and you should be fine.
You also may want to consider taking a soulgrinder or two instead of the
DP's. The soulgrinders are very durable, are good in
CC (which your army lacks) and will also draw alot of the heat off of Fateweaver. Grinders can lock up units in melee very effectively, and their AV13 makes them difficult to destroy even with a powerfist. Their fleet makes them very likely to get into assault even with a bad deepstrike roll.
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Now, if you are talking about Tzeentch space marines, you can make a very solid list with 2x Tzeentch
DP's with wings, warptime and whatever else you want. Then add in about 4 squads of Thousand Sons in rhinos, and a couple defilers or vindicators or obliterators, and you should be good to go. (Don't forget to give the aspiring sorcerers warptime or Gift of Chaos for extra fun!)
The advantage to going this route as well, is if you take two
DP's, and a defiler, and some lesser demons, you could use Fateweaver as a
DP, a soul grinder as a defiler, horrors as lesser demons, etc, etc, and you can basically get two armies with alot of models that can stand in, making both armies a little cheaper for you, and still giving you alot of variety on the tabletop.