I like the idea of having a “mercenaries” list that could work with any race, bar extremely xenophobic or hostile races (most Space Marine chapters, Necorns, Tyranids). Grovevians, Sslythe, and other suitably disgusting mon-keigh. The army would take a leaf out of the new Chaos Space Marine codex— certain HQ choices make make certain units Troop choices. That way, you can get highly customizable mercenary contingents.
Weapon systems would be a hodgepodge of Imperial and xenos technology. For example, Sslythe have a close trading relationship with the Dark Eldar and so they would be equipped with Shard Carbines and skimmers. Another race, say the Phyrix, would have heavily armored transports (AV 13/12/11) designed to punch through enemy lines and deploy brutal short range/close combat shock troopers. The Grovevians bounty hunters would infiltrate/outflank with teargas and other non-lethal weapons that ignore cover/most types of armor. I imagine that Fast Attack would be populated with things like dune buggies / air superiority fighters.
Ideally, there would be units that would behave like Lone Wolves or Marbo— hardcore bounty hunters and assassins that exist outside of normal command structures.
Statwise, I think that the army should be made up of f***ing bad***es. Naturally, the specifics would be determined by what kind of HQ you chose— you pick a Ssylthe HQ, you get a BS/WS4 T5 S5 trooper with 5+ and Feel No Pain. They would be the kind of xenos that Space Marines were engineered to fight and still have a hard time killing. Leadership should probably be low in order to represent non-ideological commitments.
The mercenary rules would further be designed such that they operate differently from the traditional Allies chart. Specifically, they would always be treated as desperate allies but count as scoring. Further, a mercenary army that is a primary detachment cannot take allies— these murderous cutthroats are on special assignment.
Overall, I think that the running theme here is an army of physically above average (MEQ) races that for whatever reason have had a difficult time competing on a galactic scale. These detachments could get extremely pricy with all the hardcore troopers and wargear, but you get what you pay for.
"Atrocity is recognized as such by victim and predator alike, by all who learn about it at whatever remove. Atrocity has no excuses, no mitigating argument. Atrocity never balances or rectifies the past. Atrocity merely arms the future for more atrocity. It is self-perpetuating upon itself — a barbarous form of incest. Whoever commits atrocity also commits those future atrocities thus bred."
"All governments suffer a recurring problem: Power attracts pathological personalities. It is not that power corrupts but that it is magnetic to the corruptible. Such people have a tendency to become drunk on violence, a condition to which they are quickly addicted."