One fact that people overlook is that a unit only needs a single model with the Blood for the Blood God rule in order to generate blood tithe points. This means that a character from a Khorne Daemonkin detachment can attach themselves to another unit to gain blood tithe points from that unit's kills.
As of now, the only allied choices that would be eligible are Chaos Space Marines, Black Legion, Crimson Slaughter, and Renegades. Daemons are out because of the Daemonic Instability rule. Characters with a mark from a chaos god cannot join units with a different mark, but there is no rule preventing a herald of Khorne from joining a squad with any kind of mark. This leads to some interesting combinations.
Whenever I ally two armies, I always ask, "What roll needs to be filled?" In the case of Khorne Daemonkin the answer is almost always long range shooting. Most of the units that are going to be the biggest benefit to a daemonkin army are going to be able to threaten an enemy from afar and potentially break open transports before your main force hits them in close combat.
One of the biggest weaknesses of Obliterators is their leadership. Mine tend to run away at the drop of a hat. Attaching a herald will not only allow them to gain blood tithe points for their kills, but it will also make the squad fearless. In return, the Herald gains a few 2+ save bodes to soak up some shots and a majority toughness of 4 (or 5). The unit will still be allowed to deep strike, and since everyone is a daemon, any instruments on your other squads can help the obliterators and herald come in more reliably.
Havocs are not as popular as Obliterators, but they can still provide some effective long range shooting.
Renegade blob squads:
Cheap troops in need of fearless, but these squads are a bit unreliable at hitting the broad side of a barn. They also don't get enough heavy weapons to generate too many kills. If you want some lascannons or autocannons, then I would grab the havocs or support squads instead.
Renegade Support Squads:
Lots of heavy weapons, but no buffer wounds. Probably a better location for a herald than a blob squad, but also more likely to die.
Renegade Field Artillery Battery:
Mortars are fun, and they can be hidden out of line of sight. Daemonkin don't lack in the department of killing infantry, but being able to threaten units out of line of sight at long range is important as well. These guys do great work and are a good choice.
Renegade Rapier Laser Destroyer Battery:
By far my favorite unit in a renegade army. These are dirt cheap and have a habit of destroying multiple vehicles per game. This is also one of the best compliments to a daemonkin army that lacks serious anti tank firepower.
Daemonkin are a fun army, and they have a way of sharing that fun with their friends. Every army has weaknesses, and using allies can help alleviate some of these issues. With a bit of planning, you can ensure that your most potent allied units have the opportunity to generate blood tithe points the the rest of your daemonkin force.