Except all the Transformers movies are like well over two and a half hours with virtually no character development, and the robots have almost no characters to speak of.
Ideally, here are the main groups of characters you need to develop, if we are sticking with the classic definitive characters:
Heroes:
He-Man/Adam
Teela
Duncan/Man at Arms
Randor and Queen Marlena
Battlecat and Orko as your buddies/sidekicks/comic relief
Sorceress
Villains:
Skeletor
Evil-Lyn
Beastman (Skeletor's creature breeder, beast master)
Tri-Klops (Skeletor's gadget, tech guy)
Hordak (Skeletor's mentor/benefactor of power)
A few other evil warrior Mooks for some color to the baddies. Like 2-3 more. Pick from the likes of Mer-Man, Trap-Jaw, etc.
You can do some big picture world building stuff with King Grayskull, Hordak and the Horde, King Hiss and the Snake Men.
I would like a prologue ideally to start the same way as the 2002 animated series. Randor and the Masters protecting Grayskull from Keldor and the Evil Warriors. Keldor gets the acid on his face, and Randor is named new King of Eternia by the Council of Elders. Forming the light and dark hemispheres, etc.
IMHO, it's just an easy way to get started with the story, put Adam on his hero's journey and just reintroduce audiences to this world. Adam can go to Castle Grayskull to learn about his destiny. Just say he is powerful because of his own naturally powerful family bloodline and that gives him the ability to channel the collected power of the other Sword of Power wielders: He-Ro, King Grayskull, Wun-Dar, Vi-Kor, Oo-Lar. Just reference and acknowledge them to streamline this.
My idea is that Adam can't just get all of his power just from the sword. It has to come from within himself as well. That way, while the sword is powerful, it's not something that anyone can use. That way Adam has to struggle and adjust to using it.
And then over the course of the films, we will get flashbacks showing Keldor's corruption and making a deal with Hordak. Grab the scene from the animated series, where Keldor makes a deal with Hordak and gives him the Skeletor head. Skeletor thinks and tries to be pragmatic, but the skull head sort of drives him batty. The Justin Marks script was bad because it basically made Skeletor a joke. He was sick and ailing and barely holding himself together. It didn't take much for Adam to defeat him.
On the villain side, I'd like for them to push the romance between Skeletor and Evil-Lynn. It doesn't have to be a major thing. It can sort of be a Mystique/Magneto thing. But Skeletor's insanity might push her to question what side to take. Again, the animated series did a great job of fleshing out Evil-Lyn's backstory, her relationship with Keldor and also her own father.
On the hero side, your main trio is Man at Arms, He-Man/Adam, and Teela. You can have other heroes, but that's the main group and relationship dynamic. I'd really like it if Orko was in there. Just tone him down a bit from the cartoon. Make it dramatic in that he has trouble controlling his powers since he lost his wand, but he can come through when it counts. I think use Cringer/Battlecat, but he can't talk.
But then again, I'm not David Goyer.