I'm really intrigued by a of this! It does seem like the early game is missing. From the short E3 Campaign tease this year, it looks like the first mission takes place on a derelict/wrecked ship, in classic Halo form. I'm wondering if its wreckage of the Infinity, and perhaps that's where Chief finds The Weapon? Then they get back down to the ring and that's where the E3 2020 demo starts, with the pelican crash landing.
I guess from there it's then just a matter of surviving against the Banished and defeating Escharum, which accidentally reveals the stuff with the Harbinger/Endless. I'm definitely okay with that structure! I really hope the Harbinger/Endless are intriguing as an enemy and that their tech brings something fresh and original to the universe. I like Halo when it's dealing with more mysterious things like this. I'm also excited to see if there is any connection with the Primordial and ancient humans. This is also the perfect opportunity to bring back the Flood if they're gonna do it, given Zeta Halo's past.
It seems like it's gonna be a nice mix of the big scifi mystery/intrigue overarching plot wrapped around a smaller character plot with Chief and new Cortana's relationship. I hope it delivers!