Firstly, a lot, and I mean a LOT of players will be returning players. from the massive player drop from Reach to 4, to MCC, to 5, and so on, a lot of old fans took a look at the E3 trailer and are going to be jumping back in (hopefully). And with that, Halo Infinite can't rely on predetermined lore like 5 did. Halo Infinite's story need to be easily understood by anyone who plays. Even I had a hard time following Halo 5's story at times. If Halo Infinite can tackle this, I won't have any doubt in my mind that they will be able to deliver in other places of the campaign.
They did emphasize that this game will be a good place for newcomers to start in their E3 2019 blog post so I think we're good on that front.
Being a "spiritual reboot" I see something akin to Halo CE happening here. Start off in the middle of a war, game takes place on a Halo ring, etc. Those who've played the series since the beginning will know what led to that point but newcomers will basically go through what we did with CE, with all the previous games being like Reach was to CE.
This leading to the point that I don't think we'll see anything crazy in this campaign. Just a cash in on CE's nostalgia to restore good will with the community (not to say that it will literally be CE plot wise, just that the game will be very limited story wise, just like CE was when compared to 2). As much as I'd love to see Tom and Lucy, the Spirit of Fire crew, Grey team, Blue Team, Osiris, Jun, the crew of the Infinity, and everyone else I'm missing team up Avengers style, I really don't think we'll see any of those characters until later games (if at all).
Edit to add where I hope the campaign will go since that is OP's question after all. Basically two things. First, everybody from the extended lore makes an appearance, particularly the Spirit of Fire Crew, Chakas, the Librarian, and other stuff from the Forerunner trilogy given that's my favorite area of the lore (Maethrillian appearance anyone? It is home of the Domain after all...) However, I realize this is not newcomer friendly at all and won't happen. Second, NO FLOOD. Seriously. I don't know why everyone wants the Flood back so much. I'd be fine with a very limited role like in Halo Wars 2 however if they become a galactic threat again then it renders the ending of Halo 3 pointless similar to how the Star Wars sequels and Terminator: Dark Fate rendered their classic predecessors pointless. I think were the Flood to return in a major way it would actually anger fans for the same reasons fans are (rightfully) upset with the two franchises I mentioned above.