Warhammer 40,000: Space Marine/WMG

Khorne is responsible for Captain Titus' resistance.
Consider: As Khorne's (unknowing) Champion, Captain Titus is therefore granted resistance to Warp power, the ability to keep on fighting by killing foes in the bloodiest way possible, and is consistently tested for worthiness by Khorne's Bloodletters.
 * Khorne cares not from where the blood flows, so long as it flows, think of how much Ork, human and daemon blood is spilled by this one Marine if he is not stopped.
 * The ability to regain health through enemies' death is often demonstrated by Titus, who ends up covered in their blood as a result.
 * The Bullet Time that allows him to kill much faster is called Fury mode for a reason.
 * The Big Bad is a Sorcerous Overlord, whose cowardly and/or sensible tactics of staying at a distance and holding enemies down with witchcraft is anathema to Khorne's nature.
 * Resistance to magic is a well known trait of Khorne's chosen such as Bloodthirsters and Kharn the Betrayer.
 * Given that Khorne has a history of empowering the enemies of his servants when he thinks it will lead to greater bloodshed (the Fire Warrior novel, for instance), this theory has a great deal of weight.

Captain Titus is a blank.
That's why he was able to handle the warp device without being injured/killed. It wasn't because he had been corrupted been corrupted by the ruinous powers, but because his being a blank makes him innately immune to the warp.
 * They would be able to tell, namely the fact that any of the pyskers the Chapter has would freak out the second they see him.
 * As far as I am aware the Ultramarines don't have that many psykers, the only ones would be Librarians and they're pretty high in rank. Much higher than Titus could be reasonably expected to have met in person. The only Chapter thus far confirmed to have a large number would be the Blood Ravens.
 * He's the captain of 2nd Company. He should have a librarian permanently attached to his command, never mind the fact that they're all psychically screened before entering the chapter.
 * Also they still need Astropaths and Navigators for there ships.
 * He's a very weak blank? I mean most blanks are an anti-magic field. His ability, assuming he is one, appears to only affect himself and only reduces the effect of psyker abilities.
 * Blanks can't think creatively.
 * Is there any actual evidence for this? I don't recall it being canon at all.
 * Jurgen
 * To clarify, Jurgen would fit as a milder version of an Extreme Doormat. He's completely obedient, oftentimes oblivious to the big picture (instead focusing on the task of 'keep the Commissar functional'. He can anticipate things, like Ciaphas forgetting his snow goggles, but I'm fairly certain that there has been few if any instances of Jurgen being genuinely creative. Initiative, yes, but creativity? Don't think so.
 * However Jurgen is only one Blank and neither the Eisenhorn series or Dark Heresy suggest that Blanks are any more or less creative/intelligent than a normal human.

Captain Titus is a Living Saint
The GE is funneling psychic/holy power through him.

The Ork attack was Staged by Chaos
The Ork invasion was caused by Chaos in order to force Drogan to use the Psychic Scourge. In addition, the device was not sabotaged but was working perfectly. Nehmeroth simply allowed Drogan to think it was viable by manipulating the warp to kill the test subjects the Scourge was tested on.

The sequel will have you meet up with the Blood Ravens from Dawn of War.
Inquisitor Thrax sends Titus, who is resistant to Chaos and the Warp, to the Eye of Terror, which is the biggest Chaos hotspot there is. While there, he runs into the Force Commander, who is on his 100-year Penitent Crusade. Together, they fight the forces of Chaos!

The sequel will have Captain Titus as a Deathwatch Space Marine.
Quick bit of fluff: The Deathwatch are Space Marines hand-picked as the combat arm of the Ordo Xenos, an Inquisitional department. They operate in smaller teams with specialist wargear and sometimes xeno-adapted technology. They are literally Space Marine black ops. In short, Deathwatch are basically the equivalent of Force Recon Marines or Navy SEALs to the 'normal' Space Marines. They are Badass without a doubt.

At the end of the game  who are canonically known to source their Marines into the Deathwatch. Also note that, meaning that rather than  , this was instead a   for the Captain, who had just ripped his way through an Ork WAAAAGH! and hails from a Chapter which specialized against fighting Tyranid, another alien species. Therefore, Warhammer 40,000: Space Marine is actually prologue to the real meat of the story, which is Captain Titus serving as a Deathwatch Marine, possibly leading a Badass Crew, letting the story expand out to include the more flavorful Space Marine Chapters such as the Space Wolves or the Dark Angels (knowing the feud between the two Chapters, having both in his team will be... interesting to say the least). Gameplay will possibly take place in a Hive World, since we've already seen what a Forge World is like, and the tight, cramped quarters of most habitation areas are perfect for the close combat gameplay style, especially when combined with the known architecture of the more iconic Imperial Hive Worlds; high spires, perfect for Assault Pack combat, long boulevards for a Devastator to really unload his ammo and perhaps the highways for a vehicular section.

Dream Team for the Deathwatch game:

Ordo Xenos Inquisitor as 'Team Leader/Mission Control', withCaptain Titus as the 'Sergeant/Combat Leader' and a Blood Raven (possibly a Librarian) as his Lancer, a Black Templar Chaplain as their 'Cleric' and (unusually for the hot-headed Knight Templar chapter) The Spock, a Space Wolf (Apocethary) and Dark Angel (Techmarine) as the Red Oni, Blue Oni Those Two Guys. Expect lots of banter. Having a Salamander as The Big Guy (Salamanders are known for heavy weapons specialists) and a White Scar as the Crazy Awesome Death From Above delivery service with his Assault Pack or Bike (if he gets a SM Trike, we have the wheels for the vehiclular section) will round off the team.


 * Makes sense, sequel wise and story. They've already done chaos and Orks and it would be dull to fight them again in a sequel. They could throw a new race into the mix, with Titus taken away by the Inquisition to join the Deathwatch


 * Tau Empire would be great for variety and gameplay reasons; their shooty army list means that its actually reasonable that a Space Marine captain would be able to rip his way through their troops, since its entirely possible for it to happen on the tabletop game. For actual combat, the fact that the Tau Empire employs auxiliaries means that we can have a huge variety of enemies. the Fire Warriors as a Shoota Boy analogue, while Kroot (primary close quarters enemies), Vespid and Gue'Vesa (traitor humans) make up the close-combat troops. If you add in other races used by the Tau into the mix, then you can have a massive variety of enemies to slaughter your way through.


 * Tyranids would have similar variety, with their mutations and all that meaning that there is no reason why they can't simply say 'this is a new mutation based off species X'. They're also zerg-styled in their gameplay, but aren't humanoid making the animation of these guys awkward, especially for the grappling style executions. Seeing Titus literally dismantle a Genestealer would be awesome, though, as would having a 'rearguard' mission involving a devastator on a flatbed truck analogue, crates of ammunition and us finding out that Titus is a horrible driver on the same level as Commander Shepard ("You know, when I was a novice I aspired to be a Land Speeder pilot, but apparently... I wasn't good at it.")


 * Space Wolves don't have Apothecaries. They have Wolf Priests instead, which are a more Badass version of a Chaplain that can ALSO heal. They'd be more likely to send a Lone Wolf of a Wolf Guard Battle Leader.
 * They wouldn't send a Lone Wolf as a Lone Wolf's entire job is to run ahead of the Space Wolves and kill the biggest thing on the battlefield or die trying. They'd be more likely to send a Wolf Guard (Veteran equivalent), Wolf Priest as Wolf Priests are their Chaplain and Apothecary equivalents or even a Long Fang (Heavy Weapons specialist and among the oldest Space Wolves).

Inquisitor Thrax is a Daemon
in this piece of fluff from "Realm of Chaos: Slaves to Darkness," way back from the early days of Warhammer 40000 in 1988, an Inquisitor named Thrax was investigating heresy within a Space Marine chapter when he became possessed by the demon living inside the Chapter Master's desk. Is this the same Inquisitor Thrax? Did that bit of fluff occur before this game? Is that even still canon? The world may never know. Or we'll find out in the sequel when he shows up as a boss. Only time will tell.
 * Nope. THE FULL PAGE of that piece of fluff shows that he didn't get possessed, just killed and eaten. Still, looks like Thrax may be doomed, but even then it would probably be after this game/any sequels he's relevant in
 * The person getting eaten in the image is the scribe. The fluff heavily implies Thrax was possessed by a daemon.

Sidonus isn't dead
They called the medic, and he was only stabbed through the chest. He's going to be a Dreadnought.
 * They called the Apothecary to harvest his geneseed, for the creation of future Ultramarines. Dreadnought isn't impossible, though.
 * Both geneseeds would have been removed by ten years after their implanting, wouldn't they?

Leadros will be a protagonist in a future sequel
Titus' words will have gotten through to him, and he'll be driven by his shame to atone for his sins against Titus and the Ultramarines. He will be a seargent of his own squad both fighting the Imperium's enemies and trying to locate Titus, so he can rescue him from whatever fate Leandros got him into.


 * Most people hate Leandros. Glad some people still thinks he's good enough to atone. The optimism feels warm!

Titus is the descendant of the God of War

 * Impossible, Khorne doesn't procreate.