Other than vehicles and Squiggoths (hoping there aren't too many Greater Knarlocs in the Front), I'm not sure what massive, high-priority, high certainty of death if engaged target I'd hope my squad got to see Astartes take out ahead of them (and like some quick-time events, I also might feel cheated that the game didn't think I was good enough to manage), but once, or maybe twice, but well spread out, I think it could be very cool to be moving up on a critical objective, only to find that the enemy is way worse than planned, and now fear is running rampant in the Guard, but then a drop pod slams in, or Termies appear, as if sent by the Emperor, and take out some Hive Tyrant/Avatar analogue, and inspire the Guard to stand their ground, and mobilize on the objective. They might live, they might die, we might even get to see them walk off the field, and know that they were awesome (WE still won the battle, of course )

I also wouldn't mind a little Space Marines reverse. If you've ever played Warhammer 40,000: Space Marine, a game I actually quite enjoyed, you might remember the plight of the Guard on Graia. Bereft of much of their leadership, and led by Lt. Mira (one of my favorite Guard NPCs in 40k), who must stand strong, if few, against the might of Orks AND Chaos, and then Captain Titus, and his Space Marines, land, and help to liberate the planet. A fun game, if a little hard in some places, and a smidge pandering to the Astartes fanboys out there, but a game where the Astartes tried, "succeeded", but couldn't withdraw, and are now pinned down, and then saved by the Guard, would be a fun option, too.

Still, at the risk of sounding silly, it is my opinion that one of the frequent ways to make Space Marines look better is to make everyone else look worse, so in an effort to keep the players cool, I'd also say keep the Astartes to a minimum, or else the players won't seem so much less, and then why do we need Astartes? or they'll suddenly look inept, and then it's less fun. My opinion only, of course.

I'm a fan of keeping Astartes powerful, but pulling no punches with their opposition. The Space Marines should only be involved in the thickest fighting against the most heinous opposition.

If mortals are in the same vicinity and *live* in that sort of fighting, that alone should be worthy of respect. If they do well, canonically Astartes can be impressed by normal humans. It just takes exceptional prowess to do so,

While I stick to cannon loosely, I have my group playing in a star system of my creation that has been fully fleshed out over the span of a few years. We use pieces of RT/DW/OW that I've butchered into a system that focuses on the shady underbelly of the Imperium where players play characters that deal in illegal activities ranging from assassinations, espionage, bounty-hunting, black market dealings, trafficking, and other activities that will either see them become the focus of an Inquisitor, caught by the Arbites, or become the richest non-noble on a planet. Think Shadowrun, meets Cowboy Bebop, meets 40k.

I see no reason to do any conversion because if the players have enough Thrones and pull in the black market (converted requisition system from OW with pieces of the DW influence system) they have access to everything from a bow to a bolter. Anything's possible as long as they use their heads, make the right connections, and know when to cut their losses.

I'm mainly telling you this, OP, to illustrate that this fluff was made for you to change and these rules were made for you to augment. If you stick to fluff and any one 40k RPG line to a T you're needlessly restricting yourself. That's the beauty of the 40k RPG, there's so much content to be utilized by the DM however you see fit. It's like Legos, with Orks and Space Elves.

If you stick to fluff and any one 40k RPG line to a T you're needlessly restricting yourself.

I tend to agree -- with the added condition that firm adherence to any source makes it a lot easier to establish a common ground with other people. Of course, then the difficulty lies in ensuring that every participant does (1) have access to those sources and (2) actually likes their representation of the setting.

For my part, I enjoy working within firmly established borders, simply because it makes me feel like my stuff could be a part of the world propagated in those sources, rather than creating its own. But it really is something everyone must decide for themselves.

Agreed as well. That's why I tend to stay away from changing established rules from the books (and only take what I like), and lay the systems I create down before any campaign starts. As long as you're up front with your group as to what you've changed, and as long as no one is a backseat DM, or a "rulebooker," everything's gravy.

And as it pertains to working with existing fluff, I like to find a middle ground that allows the players and DM to play in the familiar settings we've all read in fluff books (who doesn't want to see Armageddon or Cadia), but also create enough in-between content of our own so there's never any real need to spend copious amounts of money on campaign splats (past the copious amounts of money we already spend on this lovely hobby). Besides the money concern, there's just something about the thought that when my players think of the 40k setting the worlds I've created and the situations on established fluff locals are a firm part of that lore.

DMing is an art, and I think that's something that fledgling DMs need to take to heart right from the get. Don't get too caught up in the books and established fluff, OP, they're only there to help guide you until the training wheels can come off.