First of all...
Secondly, I think that there are so few Metroid games for precisely the same reason that there are so few Mario, Link or Star Fox games. Nintendo is famously slow, meticulous and secretive about their in-studio games, so it's only natural to expect several years between games. Furthermore, Samus has a particular albatross that Mario and Link have actively avoided: Continuity. The Metroid series has a surprisingly rigid continuity which is made even less flexible by the fact that pretty much all of the titular Metroids were killed in the second game
. Nintendo can only find so many contrived reasons for bringing the little fuckers back. Still, Nintendo pretty much blew those doors wide open with the end of Metroid Fusion, so I'd be very surprised indeed if Nintendo isn't planning behind closed doors to capitalize on that.
On the other hand, the Metroid series also suffers from a relative lack of momentum. The Nintendo 64 era saw Mario 64, Zelda: Ocarina of Time and Star Fox 64, among other great titles. These are the games that rejuvenated and revolutionized Nintendo's core franchises, and they're easily among the greatest games of all time. Alas, the PTB at Nintendo dithered while trying to bring Samus into the third dimension, leaving a gap in the series a generation wide. A lesser series would have been killed by such lack of forward motion.
As for the movie, I think that "No Country for Old Men" would be the ideal starting point. If you want drama and action with very little dialogue, that's how you do it. "Iron Man" is another good starting point, if only because it showed how Samus can be allowed to emote from under that helmet.
Obviously, the Metroids themselves are a must, and it just wouldn't be a Metroid property if Ridley wasn't involved somehow. As for the Mother Brain, I would give her a role very similar to that of Megatron in the recent Transformers movie. She'd be a cipher throughout the film; alluded to frequently, but clouded in appearance, motive, whereabouts and ability. Keep her behind the curtain until the climax... when the sight of a giant, sentient, biomechanical, missile-spewing mass of gray matter and pure, unfeeling evil ramps the spectacle and the action to 11.
Yeah, BDS, I've got some great ideas too. For another example, I would open the movie by telling the story of how Samus came to be adopted by the Chozo. Show the humans living and working out in the stars, show the Space Pirates coming in and attacking, show the benevolent Chozo coming in to aid a defenseless human, and that's it. That's all the exposition you need in one visual sequence with a speedy, kick-ass action scene to start the movie with a bang. You wouldn't even need dialogue, just showing how the different races interact would be enough. In fact, I think the movie would be more effective if you actually had to connect the dots yourself to realize that the girl we saw at the beginning grew up to be Samus.
Last but not least, there is one thing that any attempt at a Metroid movie absolutely has to get right at all costs: Atmosphere
. You only need to play five minutes of any Metroid game to realize that the environments are characters in and of themselves, every bit as beautiful and deadly as Samus herself. Besides that, the game series has always been about the relationship between man and nature. You can see it in the way that the Federation and the Space Pirates always try to lasso the Metroids, and always with disastrous results. You can see it in the way the Space Pirates continued to harvest and use Phazon, even when it turned their home planet into a wasteland covered in acid rain. You can see it in the way the Federation tried to play God by eradicating the Metroids, only to naturally give way to another species, far more terrible than the last.
If Metroid has a consistent theme, it's that no matter how powerful we may become or how far we may travel, we are still subservient to Mother Nature, and she is one vindictive bitch.