For smaller games, it could be tough to get good synergy and take advantage of Warscroll Battalions. But I think for Khorne, a good start is a Bloodstoker, Bloodsecrator, either Blood Warriors or Bloodletters, maybe a Khorgorath (or 6!). For Tzeentch, I like Flamers, everyone likes Skyfires, and a Herald on Disc and/or foot (but Gaunt Summoner is good too), Chaos Warriors or Horrors.
More advanced elite units for Khorne I like are Skarbrand, Skullreapers, Skarr Bloodwrath (he comes back to life quite often!). Building a Khorne Daemon Murderhost would also be a good goal. Lots of good Battallions in the Blades of Khorne tome.
For Tzeentch, if using Skyfires get a Shaman too, Tzaangors are pretty solid; for schemey-Tzeentchian nuanced situational play you could use the Curseling, Familiars, and Fateweaver in tandem to steal enemy spells.
Since you'll not be a mono-Allegiance, might as well get a Sayl the Faithless or convert one from any robed model (don't forget to make a Nightmaw-basically a 3 headed spawn). I also recommend a Sorcerer Lord on mount. Those two together, plus whatever your general can dish out with his bonuses, can buff up a unit of anything to do some real damage, be more resilient, and fly 18" with Sayl's spell.
For bigger games, a Lord of Change or Fateweaver are very good, and take a couple Familiars too for filler and casting buffing, which the big birds will help with also. Idea being you let them, especially Fateweaver, cast the essential spells (Sayl's Traitor Mist and the mounted Sorcerer's Daemonic Power), then the little guys can do Mystic Shield or Arcane Bolt. Launching a big unit of Khorgoraths or Bloodletters like that can be super amazing, especially coupled with the Khorne buffs of the BloodStoker and Bloodsecrator. Even without Sayl that's a good way to use Khorne and Tzeentch together.
All that said, rule of cool! Use the models you like, paint them pretty to give yourself and others an immersive experience. And timing is everything in Age of Sigmar, so don't go full head on charge if you can't make the charge!