that is a very interesting question.
i should think first off the premise to my film would be something that i have not seen in film sofar. a space-western of sorts. it starts off on earth, goes out in space. and jumps back and forth between a few planets in our solar system. set in the future ofcourse when mars and ganymede are colonized. it would be very little technology related. primarily a romance, hero legend reborn in modern times roughly related to the beowulf saga, with some political undertones between the differing governments throughout the solar system.
It would definitly be a large scale film with a fare amount of CGI, but i would demand no less than a top notch performance from the cast who would be veterans in the field with very few newcomers, i doubt i would use any high profile mainstream actors in it since their notoriety would take away from the film, i wouldnt want it to be "just another of that guys films".
but as a fan of older films when "the method" was still concidered new by all rights, where acting was actualy acting. those kinds of skills would be paramount.
for the lead role as funny as this sounds. i would want kevin sorbo, as strange as it sounds since he would be doing zero action sequences and would be required to fullfill a role that i dont think he has touched on sofar in his career. that of the out of touch, depressed, very exposed and vulnerable recovering alchoholic with a large load of guilt issues. (not in a comical way)
for the heroine's (there would be two total) im not sure which actresses would fill those roles since i cant think of any offhand which fit the bill for the characters.
a majority of the cast would be a combination of american, european and canadian actors from various films and roles such as stellan skarsgard, gerard butler, dennis storhoi, including some actors who primarily do voiceover work like beau billingsly. im sure jack black and kyle gass would make a cameo as interstellar "truck drivers" arguing with eachother over the radio.
all in all i suppose it would be a romance, with the convenience of technology almost on par with star trek, run down anti-hero thrown into a politicaly motivated scenario where faith in oneself, and the dedication of the one you love survive the test both literaly and figurativly.
the thing that i would find most important though is that, the storyline would not be intensly detailed, because you ruin the film if you spend too much time .. or too little time describing what the backstory is pryor to the film. thats why the film would probably end up as a two parter. This would enable the actors to actualy do their job, nothing ruins a scene like hurried dialogue, especialy in a romance.
i havent really thought on this concept too much until i read this post, but i think the opportunity would be very interesting, not only that in writting it i would be able to meet the actors and developers of whom i have a great sense of respect for in their own right aswell as their craft. but the people who work behind the scenes that i really admire. the CGI programmers, the composers, film crew, prop makers, stage managers.. it would be rather interesting!