Both Kaiden and Ashley will survive Virmire
Saren/Sovereign's plans will involve at least one of the former 314 species.
There will be a fourth major mercenary company in the Terminus Systems.
The Fates of Master Chief's Former Companions after the Battle of the Citadel/Alchera System Incident:
Grunt will have a different name.No way in #3!! Okeer would metaphorically referr to his pride-and-joy by the same handle as the Unggoy's Reporting Name.
New Companions for Master Chief in Part II/Investigating the Collectors:
In The Last Spartanverse, the Flood was an attempt by the Precursors to weaponize the Thorian or its ilk
The Forerunners were the Inusannon
During Jack's Loyalty Mission, Chief will flash back to his experience with P.O. Mendez
The Halo Array was a previous iteration of the Crucible
At some point, John will find out his real surname.
The Collectors will be working with the Covenant Revivalists in Part II
The Covenant Revivalists will be co-opted by the Reapers in time for Part III, somehow.
In part III, when the crew of the Normandy SR-2 (or whatever it ends up being called) is on Eden Prime thawing out Javik, the Covenant Revivalists will be on Requiem freeing the Didact
The Composer was created by reverse-engineering the Dragon's Teeth
In Part III, Dino Jake will use the reporting names for Covenant husks from "When There Was A Tomorrow" and "Guilty Sparks".
Because Chief just doesn't do discrete, the Omega DLC will be performed by...
In place of a clone of Master Chief (Since simply cloning him won't be enough to create even somewhat of a duplicate of a SPARTAN-II), Dino Jake's version of the Citadel DLC will feature a different SPARTAN II presumed KIA, this one Brainwashed and Crazy.
The Leviathans will take an interest in Chief long before they took an interest in Shepard in the original Mass Effect storyline.
Cortana will make a Heroic Sacrifice at some dramatic moment.
The Librarian's bodyshape appearance also influenced the beauty standards of the Quarians and the Asari
The Ghost of Elysium was...
The Angel of Akuze was...
The Monster of Torfan was...
Master Chief will survive the attack that destroyed the original Normandy
Most ODS Ts will be either on the N track, or the B (Heavy Assault, which is Ashley's) track.
In addition to Turians, N School and subsequent training courses on the track will include Sangheili drill instructors too.
Speculations on Chief's Love Interest
So, assuming the screwdriver that Master Chief is straight, meaning that Kaiden and Steve Cortez are out (and ignoring that in canon the SPARTAN-I Is had their sex drives physically removed to make room for more combat enhancements), any speculation on who his love interest will be? Here are some pros and cons for each possible candidate:
Ashley:Pro: Fellow Marine. Atoning for her grandfather being the first person since the Covenant War to surrender to aliens is something that might make Chief take her more under his wing. Con: Those pesky anti-fraternization rules, plus she's rather obvious fangirl, which is the sort of thing that might turn Chief off.
Liara:Pro: The only person in the original Mass Effect close to Chief's chronological age. Dino Jake has also implied romancing her, so he might be biased towards her in particular. Con: She's still an alien. Plus, in the first game she's incredibly naive, and in the second she's a tad too driven to get Ferron back to be particularly emotionally available. By the time she's ready for prime time during the Reaper Invasion, Chief may have moved on.
Tali:Pro: She had as good a reason to crush on John Sierra-117 as she did on Maleshep. Plus, they could theoretically bond over their mutual needs to be fully dressed in encounter suits. Con: Unfortunately, that crush may have been killed by Chief's adherence to Alliance secrecy rules, and then there's the fact that she's an alien, too. On the other hand, maybe he can make it up to her... Which he did, so this is right back on track!
Miranda:Pro: She's naturally charming, when she wants to be, plus, Chief, being a SPARTAN II, is no less modified than she, so she has much less reason to be insecure around him than she was towards Maleshep. Con: Unfortunately, she can be a real brat when she doesn't get her way. Plus, no thanks to the Arbiter, her reputation proceeds her to Chief, and not in a good way, and not just because she's TIM's right hand woman. Also, Dino Jake isn't exactly a big fan of her.
Jack:Pro: She's basically everything Chief, well, Linda, Kelly, or Jaye anyway, could have been if Reach had been as staffed with @$$#013$ as Pragia and the Covenant hadn't invaded, keeping the Insurrection more of a shades of gray conflict. That sort of thing is bound to cause some shock to him once he's been to Teltin, and maybe find some common ground. And with her biotics, she's one of the few people who could get into an escalating argument with Chief and hold her own, for a little while, at least. Con: She has more and bigger issues than a lifetime subscription to Sports Illustrated or Time. Even with You Tube playthroughs to help you through it, getting through her romance successfully is a razor-edge challenge with no margin for error, and Chief isn't really Shepard.
Samara:Pro: All of Liara's good qualities (from Chief's perspective) plus the seasoning of age and experience. Con: But she's still an alien. Plus, she'll still see reason to shut Chief down if he starts making the moves on her.
Kasumi:Pro: Well, it's not totally impossible. She'd have as much reason to be a Chief fangirl as she is Shepard's, at least in Game Two, anyway. And this troper suspects that Kasumi will find that Chief probably looks even better working out than Jacob. Con: Well, considering that she's an utter and unrepentant kleptomaniac...
Femshep:Pro: Well, if she exists, then unless she has the Sole Survivor service record and was stuck on Earth, Reach, or Arcturus Station shuffeling e-paper during the upcoming Battle of the Citadel, she'll be the third most decorated individual in the Alliance alive (After Chief and Anderson). Definitely closer to Chief's equal than Ashley. Plus, each origin has something in common with Chief's. Con: Those anti-fraternization rules will still be in force.
Other possible love interests, some more cracky than others:EDI Cortana in Eva Core's Body Captain Lee Riley Aria T'Loak Matriarch Aethyta Dr. Chloe Michel Gianna Parasini Emily Wong Helena Blake Nyreen Kandros