I would prefer a TV series. But a good movie can be made based on the first game. Hell you could probably make a trilogy out of the first game alone.
I haven't given much thought to the structure, I'll read the OP a little later, but the main plot of the original game is pretty darn perfect as it is, I don't see the need for major revisions. There's always going to be quibbles about whether it would be right to have Shepard as the main character because Shep is defined by the players and the actions they take as the protagonist. But if it were up to me, I'd have a male paragon Shep. How many 'renegade' characters ever play the main hero? Not often, so Shep in this game would be a good guy but will take tough action where necessary. The plot would follow that of the game. Maybe have some more background to Shep included. I wouldn't omit any of the main characters. I would include them all but give most of the screen time to a select few. So my Kirk and Spock would be Shepard and Liara. They can hook up as well as Liara would be my Shep's preferred romantic interest although to be honest, I wouldn't place a huge amount of emphasis on the romance side. I think it's overplayed by the fans and won't work too well in a stand-alone movie. A proper romance can develop over a trilogy (e.g. Han and Leia), not squeezed into one movie.
What I wouldn't want to see is a faithfully adapted movie based on ME3. Seeing the Star Brat on the silver screen and his bullshit explanation of the Reapers would piss me right off.