im going to go ahead and say Night Lords...they are definately not "loyalist"...but they are not dedicated to Chaos either...
That's not really relevent at all in this regard. Personally I very much doubt it would the Night Lords, Mcneill's hint told us that a group of Astartes from a traitor Legion, having been seperated from the core of their Legions and Primarchs for over a century and have to face the terrible decision when news filters in of their Legions treachery; do they stay loyal to the Imperium? Or do they join their Primarchs, even though they havn't been party to events which led to their fall?
What I would find most interesting if it was Astartes from say the Emperor's Children or Sons of Horus themselves. I think those two Legions would pose some of the greatest potential to a plot like this.
we know as well..the Night Lords had a presence on Terra during the Seige, yet not as large as the other Legions...
Again that is totally irrelevant. A contingent of Night Lords were present during the Siege of Terra, so what? That doesn't mean that a contingent of Night Lords were on Terra prior to the Heresy erupting, and throughout it's course. In fact the only traitor Astartes that were on Terra that we are currently
aware of was a small contingent of Word Bearers under the command of Sor Talgron.
...wishfull thinking i know, there are Night Lords novels already in the pipe-line (though not HH time frame) so its unlikely they are to star in another novel so soon?...
The Night Lords will feature in the next HH novel; Age of Darkness
. In the short story Savage Weapons
by ADB. Other than that there probably will be a full length Night Lords HH novel at some point.