That is an excellent start for an Age of Sigmar army! I recommend sticking with Khorne for now and get the new Blades of Khorne Battletome. It really supercharges the already potent forces of Khorne.
Also worthy to note, is your unit of Flesh Hounds is Battleline since you'll have the Khorne Daemon Allegiance. That means you can use all your Bloodletters in a single mighty huge unit in 1000 point games, or split into 10/20 for up to 2000 point games.
Bloodcrushers are really good I think, so if you want to maybe add anything (and stay Daemons) that's a good choice. More Flesh Hounds are good too.
Tzeentch is of course very strong, but I think with the new Blood Tithe and other Command Traits/Daemonic Gifts this will be a great army as pure Khorne Daemons.
For adding non-Daemon models, again I recommend sticking with Khorne, and get a Bloodstoker and Bloodsecrator or two or three in there to add movement to your Thirster, and attacks for all models within range of the Bloodsecrator(s). Slaughterpriests are also handy sometimes.
If however you wish to add cowardly sorcerery to the mix, as they can certainly make things absurdly powerful, Sayl the Faithless with his pet Nightmaw (easily conversion made from old sorcerer and random spawn bits) and a Sorcerer Lord on mount would be awesome to use upon a big unit of Bloodcrushers or Bloodletters. You would of course lose all the Khorne special rules from Blades of Khorne, but fun to try.
Lastly let us not forget that Archaon (big and little horse version) has the Khorne and Daemon keywords. Just sayin'