Big Battle: Multiple Character Viewpoints

Idk Ultramarines have lots of successor Chapters so that's the main reason I'd like to start with them. However 10000 years is a long time so they can be different then the original Ultramarines: 1; they are fleet based so draw recruits from multiple worlds and 2: while they respect codex Astartes they are dont follow it as closely as others 3: make them more focused on anti Xenos missions so they aren't as good with chaos battle but do well against 'nids. Also we can have additional Guard and Space Marine chapters show up as the story progresses.
Not sure who the Chaos Marines should be tho any ideas?

I'll handle the Chaos Marines. Probably an Undivided warband with ties to the Black Legion, and probably lots of cannon fodder mortals with a few high quality mercenaries who can give Stormtroopers a run for their money.

Also, how strong are Marines going to be? We need a baseline, because they've been depicted as just enhanced humans, and gods of war. Personally, I prefer the gods of war approach, with one Astartes being a match for a squad of mortals in close quarters fighting. However, tanks and range can do a lot to negate that advantage. So mostly, Astartes are a concentration of force, applied to a very specific task.

Hey guys, I'm really digging whats going on so far. That said I'd like to start organizing these different ideas into creative teams to narrow things down a bit. That said I'd like to appoint a team lead to start moving things forward. Any volunteers?

Might I suggest having a single Raven Guard ship in the area for some reason? Perhaps the world was close to where they were? I have a really good idea for a squad of them for this and would love to write about them.
Also, we need the Inquisition and Grey Knights here. No chaos invasion is complete without them, unless we're planing on having the lotalists get totally stomped before anyone shows up, which it doesnt seem like.

Raven Guard works just fine. Here's how I envision things (Obviously subject to change).

As part of a crusade in the Eastern fringe, Imperial forces attack a Chaos held world. Initially it is only Imperial Guard and a single company of Astartes assigned to this planet, as it is not believed to be strongly held. However, they run into heavy resistance due to a much larger occupation force. Scouts report that the Chaos forces are actively searching for something, and so the Lord General alerts the greater Crusade. Reinforcements are dispatched, consisting of more Guard Regiments, a convent of Sisters, a small force of Grey Knights and half a dozen Inquisitors (Likely coming along because they want to, not on orders). Meanwhile, the initial force finds themselves cut off from their landing zone, and are on the run seeking out a new place for the reinforcements to arrive while raiding supplies and fearing themselves tainted for using them. Once the reinforcements arrive, the Chaos forces call in their own reinforcements (Likely calling in favors, or promising favors in payment) and an all out war erupts.

One of the inquisitors investigates a cult that doesn't have the markings of Chaos, and learns that they are Genestealer hybrids. Meanwhile, psykers start going nuts and the Warpgates Chaos is using become unstable. The Nids arrive in full force, and everything descends into madness and carnage.

I have no ideas past there, nor who wins. Obviously, lots more needs planned out, but that's the framework I was envisioning.

Works for me, though I figure we should give it a couple days before finalizing anything.  That would be a good plan in general too, to give everybody involved a chance to chip in and make their views known.

As for defeating the Nids, it has happened before.  Ichar IV and Istvaan are both examples, as is the Battle for Maccragge.  The key is that ground forces engage and tie up the biomass collectors (Main forces) and deny them food while the Navy and Astartes engage the Hive ships.  The goal is to starve the Nids and force them to use all of their biomass up.  So Nids don't have to win.

I'd also want to keep the number Grey Knights involved small, to give a feeling of there not being enough of them to go around and let the Knights express their frustration and desperation.  Definitely need them to be even more powerful than a normal Astartes.  The idea is that the Imperials are the ones on the offensive, and don't yet know there's a full fledged warpspace invasion.  The reinforcements would send what they could spare, but 2 Strike Squads and a Terminator squad would probably be seen as enough for what was considered a mid sized fight without a whole lot of outright daemonic presence.