Some possibilities I'll toss out for consideration:
Riflemen
The Battle Brothers of this chapter specialize in the use of auto bolt rifles as an all-purpose weapon - they allow for rapid unloading of highly-accurate bolter fire, and are overall some of the most reliable firearms available to an Astartes. Furthermore, their expertise in these relatively simple weapons allow these soldiers to train their auxiliaries in the use of similar - if not Astartes-grade - weapons with relative ease.
Duelists
No Battle Brother of this chapter would ever be found without their power swords - and with good reason. They are masters of the blade, making strikes with expert precision to slip through weak points in enemy armor, and with the skills to exploit any mistakes their enemy might make.
Every Human's Right
The geneseed alteration this chapter made isn't immediately apparent, not even if you took one of their Battle Brothers and cut them open. Well, half of them. Indeed, it is not the marines who have been altered, but rather the range of their recruits. This chapter's geneseed, in fact, is compatible with both men and women, allowing any human to qualify for and join the ranks of these Astartes.
The Right of All Sapient Beings
One of the Prognosticators has been raving about how this was a mistake. I assure you, they were all for it while we were working on the project, but they simply will not shut up now. No matter; the deed is done. We intend to give all species who might live harmoniously equal opportunity, and now, we have taken possibly the greatest step toward truly doing so. We will be called heretics, but that was already the case. Now, anybody who might aspire, no matter their species, might join our ranks as one of the Astartes.
What doesn't kill you...
...can be brushed off, given a few minutes. We have done some work editing a few of our organs - in particular, the Biscopea, the Larraman's Organ, and the Sus-an Membrane. The first grants us our increased muscle growth, the second closes our wounds, and the third puts us into suspended animation when wounded... or, rather. That's what they did. Now, in concert, these organs can allow any one of us to recover from any injury that is not swiftly fatal, at least if given some time. We might still be knocked out for the duration of the immediate firefight, but we will likely once more be ready for action soon thereafter.
(Yes I went and looked at the actual Space Marine extra organs to figure out which ones would be most applicable to making us heal fast.)