Just did a quick write up on something I want to try to get started around, and hope people will be able to help point out the bugs!
Herald of Khorne-140
-Armour of Khorne
Herald of Tzeentch-160
Lvl 2 Lore of Death
-Master of Sorcery (Death)
Not sure what some of my tactics are, just know that the herald of khorne is going in one of the bloodletter units, seekers hunt warmachines and light armored units, the flamers provide fire support, while the tzeentch herald snipes out characters like wizards and BSB.
Not a bad start for a list. Couple of things could improve it though. First, Seekers aren't really the best warmachine hunters we've got, especially given how expensive they are. For the cost of that unit you could instead get several other units that will each be at least as useful. I would recommend to drop the Seekers, the Pyrocaster upgrade (not worth the points), and 1 Flamer (to fit what's coming into the Rare allowance). Then add 2 units of 1 Fiend, a unit of 5 Furies, and 4 more Bloodletters to the Herald's unit. Daemons really excell when we load up with chaff units and redirectors, and with 2 Fiends, some Furies, and the Flamers you can easily take out an opponent's chaff and warmachines early, then mve in for flank and rear supporting charges in the later game.
I am glad you reminded me of the Fiends. I personally wasn't looking forward to having Seekers. Not a big fan of Slaanesh. Fiends are one of the two units (other bieng our GD) from him that I like. That can definitely be done.