You can't have too many characters in a first movie. If it's only going to be 105 minutes to 2+ hours, you can only have so much screentime.
Here's what the movie definitely needs.
1. Prince Adam and Sword transformation
2. Orko
3. Hordak appearances (not major villain but setup for later)
4. Man-at-Arms as He-Man/Adam's Yoda/Obi-Wan Kenobi mentor
5. Completely set on Grayskull. No planet Earth
6. Take inspiration from the mini-comics and the 200x animated series. IMHO those should be the biggest sources of inspiration.
If you look at the early mini-comics, they are basically just illustrated short, pulp adventure stories. So try to combine that with some of the storytelling basic rebooting of the 200x animated series. The 200x animated series basically acknowledged all the previous stories but just reset the continuity, so there was on cohesive backstory and origin for Grayskull and all the parties involved.
Why do I like He-Man so much? Well for starters, I like that it almost has this look of like Conan the Barbarian. It's like old school Hyborean age fantasy, but it still has sci-fi elements mixed in. There's interstellar travel. There's technology and vehicles. That juxtaposition is so interesting to me. I think you can do a lot of fun stuff with that.
My issue though is that He-Man is a product of the 80s, and if they make a new movie that say is faithful to the source material, people will complain it doesn't adhere to their values. I'm not sure how you update it and make it relevant to modern audiences and get them to accept it as is.