Vanilla Space Marines, inevitably the yardstick by which all other armies are judged. While the various offshoot army books certainly have their own special things to offer, the basic codex has always had a lot of flexibility and the most recent one is no exception. With that flexibility comes opportunities for "counts as" armies!
For this one I'll pull out one of the armies that has been missing for a while - while their brother ordos both got nice books, the third has been conspicuously absent - that's right, the Inquisition's Ordo Xenos, the Alien Hunters!
Ordo Xenos Inquisitor (Librarian, or Tigurius)
-Power armor and psychic powers, force weapon? Check, check, check.
Deathwatch Captain (Space Marine Captain)
-Be sure to buy the Hellfire rounds. Alternatively, a Pedro Kantor counts-as would make the Deathwatch (Sternguard) squads scoring. You could also use the "real" Chaplain Cassius, on temporary assignment sharing his expertise with the Ordo.
Deathwatch Kill Teams (Sternguard)
-Tough vets, magic bullets - easy fit for Deathwatch. If use Kantor can even be scoring.
Inquisition Storm Troopers (Scouts)
-Well equipped and trained carapace armor troopers.
Inquisition Skimmer (Land Speeder Storm)
Storm Trooper Bikers (Scout Bikes)
-More Storm Trooper options.
Could be run a number of ways, but I picture the Storm Troopers setting up in various positions to harass the enemy, with Deathwatch teams podding down, sometimes guided by locator beacons on the bikers.