One of the things that the Transformers movie did well was to draw the non-Transformer fan (or casual Transformer fan) in by having a big part of the set-up to the movie being a guy who was going to purchase his first car.
Having a connection like that really helps I think.
I think that a good way to kick off a MotU movie would be to have a 10 minute bit of seeing Marlena getting into a space shuttle (maybe some shots of the "NASA" symbol), and chit-chatting with her friends, and then being dragged into some sort of wormhole, crashing on Eternia, meeting King Randor...
I'd then fast forward the movie ahead about 20 years, and aside from a couple of slight references to Earth made by Marlena, I'd leave it aside for the rest of the movie.
I find that the "human from Earth goes to a strange, mysterious world" is a story premise that never gets old, and actually can fit very well with high epic fantasy.