I'm a huge Rambo fan, but this sounds like a bad idea. Rambo has always been military action flicks, adding some strange sci-fi element just does not make sense. Also, the last movie had the perfect ending for Rambo. It brought the character to a good place after all he had been through, I say leave him there.
As a former infantry Marine who was just in Iraq not to long ago and who also happens to be a total comic book nerd I think it's a great idea. The sentiment would be lost on some but most of us would certainly appreciate it. However, I would recommend trying to get the publishers to send TPB's if possible. Imagine how frustrating it would be to get the beginning or middle of a story arc with no way of getting the issues that finish it. ^_^
But more importantly, there are only three ways to go with this movie. You make it silly and campy, like the original but with a new take and it could go either way, good or bad, but in either case I think the movie would be forgotten in a few months time.
You start from scratch pretty much ignoring the original flicks except for some major details, and piss off those of us who loved the original, but if it's good draw in a new crowd.
Or you pull off something thats serious but pays respect to the original and you just might have a movie that revitalizes the franchise. Personally this is what I'm hoping for.
As for who should play Barbarella, I'm not sure. Rose McGowan is certainly hot but definitely can't pull off being blond, it just doesn't fit her, and I think the character being blond is important as far as the iconic image of Barbarella goes.
Sienna Miller would be an okay fit I think, but I have to say Erica Durance would be way hotter in that role.
I think it's safe to say that Superboy Prime is the more powerful of the two characters. HOWEVER, the Super line of characters (Superman, Superboy, Supergirl, ect...) have been shown to be very vulnerable to magics, something Doom is extremely familiar with. Also in a battle of wits I think it's safe to say Doom can run circles around Superboy Prime.
So with the one day of prep time I say Doom wins this one hands down despite Superboy Prime being the more powerful.
Keeping in mind that Leo has shown himself to be an emotional character I think Deadpool's taunting would throw him off his game just far enough for Deadpool to do something crazy, unexpected, and devious enough to make him the winner. Remember, Deadpool may be completely insane but he is my no means stupid when comes to fighting.
I think it's a tough call and could probably go either way depending on the circumstances. If Kyle Rayner figured out the Guyver's control medallion is his achilles's heel I don't think any Guyver would last to long, but barring that Guyver One would eventually get the better of Kyle one way or the other.