You definitely need to balance all those characters; I would love Blue Team to come back and while I couldn't distinguish the ODSTs in the first K5 novel they got fleshed out nicely in Thursday War--but having all those characters together would be a mess. The Spartans require less foreknowledge I think and you can drop them in with a line or two of dialogue to Chief and figure out their relationship. K5 feels like they would need to be introduced as a unit and that's a lot of random ODSTs (unless they're Spartans by this point.)
Escalation made me realize that we've kind of got a pretty gaping hole in history between K5 and Halo 4/Ops in regards to the Covenant. Last we saw the Arbiter he was on the ropes and got his bacon saved by Infinity's Motherf'in' Pimp Hand, but Telcam didn't seem particularly defeated. Maybe Jul came back and basically assumed control of the religious zealots and left Sanghelios, leaving the Arbiter free to clean up the stragglers? Seems kind of odd we went from everyone ganging up on the Arbiter to Hood saying that he's doing swell.
Oh yes I agree. I just saw the posts above talking about Lasky saying he was the only recently-introduced character that was interesting and I disagreed, wanting to talk about Vaz/Mal/Dev. I kind of went off mentioning other characters but that's where it came from.
I agree on the problems between Thursday War/Glasslands. Traviss did good work in the second book to make them more distinguished for sure. I don't even need these characters to be anything big. Even just some minor cutscene in Halo 5 with meeting with Admiral Osman with the ODSTs+Naomi in the background would be cool.
Blue Team deserves some screen time with all they've accomplished in the books. Hell, I thought Will was the most badass Spartan ever and I was hoping for him to appear at some point but now that he's a pile of ash floating in space around Trevelyan, I guess that's out the window.
Mortal Dictata will probably fill in a lot of what you're asking about, I assume. Jul probably convinced all of those goobers on that backwater planet to join him, went and rescued Telcam+co and left to search the galaxy unknown. Hood/Infinity helped Arbiter rebuild and there we go.
I'm more interested in whether or not Parangosky actually finally stepped down or if she somehow dies and forces Osman to take the throne as head of Oni.
EDIT: And some of those characters I mentioned (Blue/Gray/Fillion/whomever) annoy me because we already have established fun characters in other games or in books and instead of them actually getting prominent screen time like they deserve, we get boring and uninteresting characters like Crimson or whoever. I know his relationship with Halo was mostly because of Bungie, but dang. Bring back Fillion.