Avengers: Infinity War

In English you always capitalise the words in the title of a book or film.

You can say "last" in this sense, but it would be more common to say "latest". "Last" would be more commonly used if it was the final film in the series, which seems unlikely.

I hadn't seen the previous films before but I was able to follow the plot thanks to a short summary video that I found on youtube YouTube which was very clear.

You still haven't seen the previous films. You would say that you hadn't seen them "before" if you had in fact watched them yesterday as well.

The film was amazing and, even if it lasted more than two hours, I would have stayed all night in that room.

I would probably say "cinema" rather than "room".

My favorite superhero is Doctor Strange because it he has very unusual powers compared to other characters such as Iron Man or Hulk.

You always use "he" for men and "she" for women. (And generally for male and female animals as well.) "It" is for inanimate objects.

I'm becoming more and more a Marvel film passionate fan and I will surely see the next chapters of the saga.

"Passionate" is an adjective, not a noun (sostantivo). You can't "become an adjective". A "fan" is someone who is passionate about something. You could also have said "I am becoming more and more passionate about Marvel films".

In the meantime, I will discuss with my friends on the possible future scenarios that can be developed in the next films.

You don't "discuss on", you just "discuss". However you can "have a discussion on " some subject. (Though even there, it is more common to say "have a discussion about" that subject.)

Your mistakes were fairly minor; that was well written and expressed.