This is just my opinion. But after frequenting these forums for many months, now, I have finally figured out what could have made Fireteam Osiris’ presence in Halo 5: Guardians more acceptable. And, of course, this opinion of mine is completely hypothetical, as the die has already been cast, unfortunately.
As it stands, Buck is the only character of Osiris that game-only fans of Halo can recognize (ODST has been out long enough, now, for most to get a chance to play it). The rest (Locke, Vale, and Tanaka) were all introduced either in books, comics, or, in Locke’s case, the miniseries “Nightfall”. Anyone who didn’t read/watch these sources therefore knew nothing about them - though even the casual-est of fans would have seen the Halo 5 trailers on TV (several aired on my local channels via plug-in antenna), and therefore would recognize Locke as being important to the story (even if that story ended up having almost nothing to do with the trailers and promos).
Based on the forums and several blogs on the internet, I’ve seen that reception of Fireteam Osiris was mixed, ranging from flat-out dislike to admiration. For me, they felt rather random. They had impact on the story, yes (assassinating Jul 'Mdama, assisting Thel 'Vadam’s fight, rescuing Blue Team from the Cryptum), but in a perfect (Halo) world, those actions could easily have been all done by Blue Team - familiar characters would receive better reception and acceptance than relative newcomers to the franchise. Instead, those actions are given to Osiris, with Blue Team having practically no purpose whatsoever (relatively speaking. Especially with only three full levels dedicated to them).
So this is what I wish could have happened: Either a) have Osiris ditched entirely for Halo 5, and instead have the Master Chief, Fred, Linda, and Kelly dominate the narrative; b) keep the story in place, but give more levels and purpose to Blue Team, so there’s a better balance; or…
C) …have Halo 5 be preceded by a spin-off game, much like Halo 3: ODST, where Fireteam Osiris gets their own game. It could have been called Halo 5: Osiris, with Halo 5 itself having no sub-title, so that it’s evident that one is a spin-off and the other is the main, numbered title game in the series. In this hypothetical game, game-only fans of Halo would be properly introduced to Fireteam Osiris and get to know the characters properly. This could work within the already established EU books, taking place at some point before Halo 5 but after the events of the books and such where Locke, Tanaka, and Vale had been introduced. Then, when they come into Halo 5 (no subtitle), everyone would have proper knowledge about them.
Of course, it would have been preferable if Halo 5 had been a proper sequel to Halo 4 - as it stands, Halo 4 and 5 both feel unrelated, like their own standalone Halo games, with no real logical connection between the two. But, alas, we have what we have. Even if we still got the shoddy story of H5, I wish that at least 343i/Microsoft had made a game just for Osiris that would have been released before H5.
What do the rest of you think about these ideas? It’s all just hypothetical, as I mentioned at the start. One can dream, though!