The ideal tone for the movie IMO would be ensuring that at every step of the way, the audience feels like they are in Eternia at all times.
This means no anachronisms, no "Whoa! Cool sword!" type dialogue, and no obvious aping of other fantasy films in terms of photography, design, staging, etc. It should be its own unique experience, and to that end they should avoid watering down or muting the character designs as much as possible.
MotU is already as unique as anything, but if you have idiots who don't "get" what MotU is about working on this project, then you start to creep into modes of thinking where the X-Men wouldn't wear silly costumes "in real life" so give 'em all black leather.
Not saying I think that was a terrible idea for X-Men, but that type of thinking has NO PLACE in MotU.
MotU is pure fantasy . . . we don't need in-depth explanations about how the Attak Trak works, or how the Sorceress' costume isn't practical enough for flight so let's redesign it.
Don't try to make MotU "realistic".
Don't mute the vivid colors that make Eternia so alive and each character look so iconic.
Man-At-Arms has orange armor. Stratos has bright red wings. Skeletor has blue skin. Deal with it.
To address the specifics in the poll options, I guess I'd like to maybe see a mix of all of those to a point, but I chose "Superhero" just because I think above all a MotU movie needs to be FUN and ENTERTAINING.
It shouldn't take itself too seriously, it shouldn't be dark and brooding, and it shouldn't be overly violent with senseless character deaths.
The problem with fans who want a more "mature" or "gritty" take on MotU is that IMO they are selling the character designs short.
Almost more than any other property in history, with MotU the characters are so iconic, they are so archetypal in nature, that you can almost immediately pick out who is good and who is bad, and what their powers and abilities are just by looking at them.
Not only does this make these characters PERFECT for the film medium (where one fundamental is conveying the most amount of information in the least amount of time) but it means that if your lead villain is a blue-skinned demonic evil sorcerer with a freakin' SKULL for a head, you don't need to artificially make that "dark".
When you have a stitched-together mechanized Frankenstein's monster with a metal jaw, you don't need to do much besides just SHOW Trap-Jaw to convey that he's someone you shouldn't mess with.
IMO, the wish for such a "gritty" version of MotU is mostly a knee-jerk response to the campiness of Filmation, and I would add feeling self-conscious that in its more pure form, MotU would not be accepted by mainstream movie-goers without any efforts to try and make it "cool".