After I saw
this, I knew I'd have to share what I do know about D23. Here's some information on what I've heard Marvel will be showing at D23.
First and foremost, there will be very little if anything at all relating to Phase 5. People seem to think that Marvel only showed Phase 4 at SDCC but will show off Phase 5 at D23 but this isn't the case. So don't expect any X-Men or Fantastic Four updates. Mahershala as Blade was an exception if only because he approached Marvel for a role he was interested in. I would love to be surprised but there's nothing planned.
What you will see is more information on the established Phase 4 properties. Many of the movies coming in Phase 4 lack concrete information on villains, side characters, etc. Don't expect footage, but there will be a lot of concept art shown for the Phase 4 films and D+ shows. Particularly Shang-Chi and The Eternals. Also, there will be some connectivity with The Eternals and MCU thus far. If you've read the 06-07' Neil Gaiman run you know what to expect. I'm also hearing we'll see what Jane Foster Thor looks like but again, concept art.
There will hopefully be announcements on casting, but this is dependant on whether they close the casting. Shang-Chi was cast days before SDCC but if they didn't cast the role they'd have to skip. Marvel will obviously try to cast the roles of Nightmare, Beta Ray Bill, Gorr (The God Butcher), The Benefactor (aka Norman Osborn), and many more. Still, if they don't have the roles filled we'll have to wait. So don't be too sad if we don't get these announcements although they will come eventually.
D+ will be the biggest showing. This is one of the biggest promotional events prior to the service's release. They are going to try to drum up support to get their base subscriber numbers up. So expect a lot of news on cameos, casting, storylines, etc. There won't be any new shows announced, but we will get to see some of these existing shows fleshed out.
There's always a chance at a surprise. Still, I wouldn't hold out hope for anything huge. From what I'm hearing they aren't going to show any 2022 and forward content until SDCC 2021. This might be disappointing to hear but Marvel felt particularly burned by Inhumans and a few other smaller shifts in their schedule. They're keeping their reveals tight and short to avoid announcing something that falls through or changes.