If Bumblebee is the heart of the Transformers, Optimus Prime is the soul.
The Optimus Prime we have seen in the movies is often the moral center of the films and the stalwart hero, so a movie focusing on him would demand a different approach.
The two characters are very different, but that’s what could make their respective spinoffs interesting, they would not feel like the same movie.
Bumblebee is a more emotional character, and that’s part of what makes his solo outing interesting.