The Red ending + the Indoctrination theory are canon, not because I like them but because Electronic Arts need to draw old fans back. This is a pretty nice way to flatter the community, inject hype, and explain why other endings are not canon. I’m almost completely ignoring the teaser and the poster (apart from the time skip and the presence of Liara), because I know trailers are often created by third parties while the story of the game is still being rewritten and corrected. (Developers themselves often laugh at this phenomenon because it is so ubiquitous. Here is a great ironic example – The Outer Worlds 2 trailer, check it out if you haven't watched yet -
link - an exhaustive 1.5 minute funny video covering the realities of game development).
I love Mass Effect but there are several reasons why a direct sequel is worrying me. However, all of them are subjective and do not claim to be true.
The “Big Evil” issue. Usually, the evil that we confront develops from chapter to chapter in order to finally challenge the main character and all his allies (we move from the “rats in the basement” fight to an ancient deity threat). In the Mass Effect trilogy, we literally have united different species of the galaxy to prevent their total annihilation. Now the story is over and any writer can fall into the following trap: they need to create some new Evil. From the writing standpoint, it simply cannot happen that after the incredible galactic threat we will be engaged in something like a petty merchant war (who are fighting for the right to become Commander Shepard's only favorite store on the Citadel). It will be boring, and silly, and not epic. But if something threatens the galaxy again, it will feel like a repetition. This story was conceived as a trilogy where the highest note was a galactic war. The only way to add scaling to it is to threaten the universe, life and matter in general, but hey, this is really difficult to write. (One would need physics, mathematics, astronomy and the ability to talk about all that in an understandable and interesting way). To summarize: I don't want to stop another boring psychopath or a race of the Milky Way invaders.
Personality changes to our favourite characters are possible. A new author, or an old one returning to the story after many years, they can easily forget the essence of these characters so they can change in an unexpected direction.
Devaluation of the victim. I definitely don't want to see Shepard alive or resurrected with the help of Lazarus. I love the Commander, but without his/her sacrifice, we would lose the beauty of the moment.