Now with that out of the way in my opinion Halo 5 is extremely curious. I honestly don't hate the ending like most people. In fact I love the ending! The prospect for this war against the Forerunner's overwhelming technology and the possibilities for what comes next excites me. I instead feel that the problem is with the events leading up to it. The game does not earn the big cliffhanger ending. I think the big problem is that they advertised the game's story at it's core is a search for Cortana and trying to uncover her intentions, but it get's sidetracked with so much other stuff that it almost completely loses focus at points and thus loses out on the part of the story that really matters.
I realize now that the hunt for John was only supposed to be a front in the first place and the intrigue was supposed to be that of figuring out what's going on with Cortana, but the game gets way too distracted with Locke and Sangelios which ultimately accomplish nothing that acting as filler, and taking up time that could have been better spent on the main story of Cortana's reappearance.
They could have easily condensed and consolidated certain levels (like Glassed, Meridian Station, Unconfirmed and Evacuation) to make room for more important things, like missions on Genesis or cutscenes that actually advance the main story, and even cut certain levels entirely like Enemy Lines and put the Kraken into the Battle of Sunaion.

I think the problem of the story losing it's focused is the most pronounced when Locke finally Catches up with John and John already knows Cortana's goals aren't exactly the best. We never got to see what brings him to that conclusion. Exuberant Waifu even states that they have been wandering the Conduit for hours. You'd think seeing what happens between then that lead John to be suspicious of Cortana would be more important to show than Mike Colter and friends going to find a pet sentinel.

I think 343 was just desperate to fit the fall of the covenant into the game and the whole thing suffered because of it. Maybe they should have waited for Halo Wars 2 or done a completely different project for the sake of giving both stories the space they need.

If they had focused more on the real point of the story, and constructed it in a way that felt less like everything had just gotten started at the end, I think the prospect of Cortana beginning her takeover of the galaxy. In fact I think the main events of the main story-line lend themselves to this very well. John finding  out Cortana is alive and has plans for this forced control over the galaxy over a mislead idea of peace and ends up being captured out of his compasion for Cortana, and then meanwhile Locke, who was sent on a manhunt for John who went AWOL, has his mission changed to a rescue when he realises the truth of what's happening by the clues John left behind. You take those elements of the story develop those without all the needless filler and you have a full narative that makes the ending woth it.

Other than that, Loke was boring. No idea what 343 wanted to accomplish by introducing him. In fact everyone  other than John, Warden Eternal and Cortana are pretty anemic or nonexistant in terms of anything other than backstory. They needed to do more with the characters that had or simply not had them at all.

John seems to be growing as a character. He has at some point between Halo 4 and now developed his own opinions and convictions including about what was done to him, but despite that he hasn't gone full Karen Traviss like everyone else (Including Halsey herself apparently) although I do question his comparison of the Spartan program to Coranta's forced peace.

Warden Eternal is an interesting fellow. He clearly shares the sentiment of the Didact and plently other forerunners that humanity are dangerous and animalistic, but still defaults his goals to Cortana. Why he belives AI's are to inherit the Mantle is unknown and an interesting prospect. It is very likely that he has an ultirior motive and may have just made the entire thing up, especially since the only other Forerunner AI we see in the game thinks he's gone apeshit and is firmly agains his and Cortana's goals.
And everyone makes white knight jokes about the Warden which is an absolutely hilarious interpretation of his charachter but ultimately what his is doing, in subverting Cortana to stop John is exactly what he should be doing in his role. He knows John only as a warrior of what he considers a dangerous and savage race, and what we gather from his dialogue, probably knows a good bunch about the Spatan program in general and even John personally. At the end of the day he knows John wont agree with Cortana's plan and will likely do everything in his power to stop her, making him a threat to Cortana and their plans.

There's still the issues of the Domain and the audio logs
As much as I do like about the campaign and  think they had some good ideas, I still think it was the weakest story in the series by far.

but that was just my 2 cents on the matter for now, so with that what are you guys' thoughts, oppinions and theories on Halo 5