Thanks for the input guys!
On the subject of garrison duty, it's been said that the Space Marines don't stick around that long on one planet (with all those other systems to free and aliens to purge, you can't really sit back and relax for a while), but what if said planet is under constant attacks of such ferocity that it requires a more permanent Space Marine presence? Would they not create some sort of outpost, whether on the surface or on a station in orbit?
Also, there's always the possibility of a Warp Storm cutting a system or planet off from the rest of the universe (often for quite a while), which would lead to several Marines being stuck on an inescapable planet and with no means to be rotated out of service.
So there's a chance that Chapter C might have some Marines left on some backwater planet from which they can start to rebuild the Chapter again, although how the surviving Marines would get to them would pose another problem...
As most of you mentioned the surviving Marines going on a death or glory run into the Eye of Terror, I've been thinking about what happens to those guys once they get there. Now before you say, "they get insta-killed by Daemons / Chaos Space Marines / Daemon Primarchs / Abaddon's incompetence, duh", I'm talking about Loyalist Marines who by chance or the Emperor's will bump into some other Loyalists.
Let's say a Captain from Chapter E did some pretty horrible things to some semi-heretical Guardsmen with his company, and he and a portion of his men get a one-way ticket to everyone favorite murderous horrordimension. Once inside the Eye of Terror, they descend on the first Daemon world they see, deploying in full force to slay dem heretical beasties. Now while they're down there, they bump into survivors from Chapter F's chapter, who had been part of some failed crusade into the Eye. Both Chapters have different reasons to be there, but Battle-brothers are Battle-brothers (unless they hated eachother beforehand for other reasons), so they at least try to help eachother out.
At the end of the battle, both forces are rather depleted, but you can't say 'Only in Death does Duty end' if you don't get to that 'Dead' part, so both parties decide to go kick some more Daemon ass somewhere else.
Now, would these Chapters part ways to regain their honour by themselves, or would they band together to fight a common threat? I'm aware that this also depends on which Chapters we're talking about, but the idea of a Loyalist 'warband' somehow appeals to me, so I'd definitely like to hear your opinions on this
Drakeslayer wrote:Ezra, you're not thinking of creating a
SM-themed remnants force are you? Regardless of the fluff I think you should go right ahead and do it.
Anyways, I digres: Remnant Space Marines, DO
IT!!!!
Alright, you've got me; I admit that it has crossed my mind

I already have a few ideas for something like that, but my wallet wouldn't survive it I think
Dicrel Seijin wrote:
The Soul Drinkers were declared heretical, because, let's face it, when your chapter master's lower half is a giant spider, there aren't a great many other outcomes.
That sentence alone deserves an exalt!