Chaos Space Marines! Similar to regular marines in many respects - power armor, terminator armor, various tanks - but different both by what they don't get (tank variants, scouts, speeders) and the extras they do get like cult troops, possessed and other daemonic goodies.
So what kind of counts-as army to make using Codex: Chaos Space Marines? Howabout some non-Tyranid insectoid xenos? Let's call them the Kakak Collective. A hardy race that relies on its natural resiliency enhanced by a technological mastery over their own genetic code partnered with advanced energy weapons.
Swarm Leader (Fabius Bile)
-Mainly included to allow for enhanced warriors, but he has his own uses as well.
Elite Cadre (Chaos Terminators)
-With powerful talons and double blasters they can do a lot of damage, and their extra-thick chitin allows them to survive most retaliation. They can also tunnel, lying in wait for the enemy until the right moment (Deep Strike).
Veteran Frenzy Cadre (Possessed)
-Sometimes after repeated activation, a member of the Frenzy Cadre will metamorphasize into a true killing machine. The metamorphasis can take on different forms, but they all involve better ways to slay the enemy up close at the cost of a disdain for any manner of ranged combat.
Frenzy Cadre (Chaos Space Marines enhanced by Bile)
-Armed with blasters and talons, when activated for battle they throw themselves at the enemy with abandon.
Deathwing Cadre (Raptors)
-Bugs with wings and guns.
Destroyer Cadre (Obliterators)
-Similar to the Elite Cadre, they can burrow as well. Armed with powerful claws and a large variable-frequency blaster, they have a settings enabling them to fell any enemy.
-An extra psychic brain-bug attached to one member of a cadre. Depending on specific breed, it can enhance the reflexes of its cadre (MoS), add to their frenzy (MoK), deaden their pain (MoN) or provide a psychic shield (MoT). They can also contact and guide burrowing Elite Cadre or Destroyer Cadre to needed locations on the battlefield (no-scatter Deep Strike).
You could of course also include bombadier-beetle Defilers, other cadre with fancy blasters for Noise Marines or Thousand Sons, transports and the like if desired as well.
Next up, Daemon Hunters!