Just kicking around an idea, since I'm stuck at work on the 4th here. If anyone likes any of this feel free to lift it for your own use. Comments are also welcome. I'd almost started my own thread, then remembered this one and thought it would fit better here.
What if one of the lost legions, say XI for the arbitrary reason that the letter X stands for eXtreme, were actually the core of the Legion of the Damned. In this incarnation, they'd be sort of the 40k Space Marine equivalent of the Night's Watch from Game of Thrones. Perhaps their original mandate was to operate outside of the Imperium's bounds as an interdiction force.
I haven't seen the new stuff on the Legion of the Damned in "Honor the Chapter" yet, so I don't know if there's anything in there fluff-wise that would cause a problem. I'll edit and clean this up as new things occur to me, but for now it'll be a bit rough.
*Legion XI - Interdictors
*Primarch - Horatius (Status - TBD)
*Role - Guarding the borders of the Imperium, "holding the line" against unknown threats
*Tech - Mix of Heresy-era equipment, sprinkled with lesser new equipment from "adopting" reinforcements; possible archaeo-tech to explain LotD resilience, etc.
*Redacted from Imperial Records - Due to the the Heresy, this Legion was stricken from records to protect it from being attacked from the rear by supposed 'allies'. (Decision possibly reached with Horatius' knowledge, possibly worked out with the Emperor. The Legion knows it can never come home, but that their service is vital.)
*Recruitment - As a fleet-based legion, the Interdictors pull recruits from any suitable world. Additionally, any Marine forces they discover that would otherwise be lost to the void (such as the Fire Hawks) are drawn into the fold if willing, or possibly mind-wiped/sworn to utmost secrecy.
*Livery/Heraldry - TBD/Legion of the Damned colors
*Strength/Deployment - Scattered thinly along the borders of the Imperium, due to the vast space covered, precise operational numbers are unknown. Depending on threat levels detected, may operate in units ranging from a couple of squads to company-strength, though this leaves large stretches undefended, or only lightly covered.
I'm sure there are other things I'll think of. I'll add them as they come up.
**EDIT #1: The "warp contagion" story with the Fire Hawks could be a cover story to ward off too much in the way of investigation. No one wants space cooties.