You have to have Garrus and Tali in ME1 - even if only as minor characters after their into - for their introduction in ME2 to make sense, not to mention the everything to do with Tali's trial. Otherwise you'd have to completely rewrite their parts. And 5 supporting characters really isn't that many. 3 even - Liara, Wrex, and Ashley Alenko (a shame but it makes sense and the VS role can be shuffled on to others in ME2 and ME3), with Garrus and Tali being minors like Joker, Anderson and Udina until they get bigger roles in 2.
And there's no way they won't have the movie be about Shepard or prominently feature humanity.
But yeah, ME1 in itself would make a great movie.
The much harder one would be 2, since the "main plot" is actually very basic and much of the game has to do with the sideplots of the 12 squaddies - which is great, but doesn't lend itself easily to a film. Of the 12 the easiest to cut would be Jacob, Kasumi, Zaeed, Samara, Jack, Grunt, and Thane. All of whom really pain me (welll... except Jacob...), but it probably has to be done. Maybe cameos as part of a 10 minute "recruiting people" section and background characters after that? If Legion didn't have a critical role in resolving the Geth/Quarian conflict he could easily be cut too. Keeping that peaceful resolution would also up the drama with the likely Destroy ending.
So that leaves Miranda and Mordin who need proper roles and introductions, plus Legion without a rewrite. Up Tali and Garrus to fairly major characters along with Joker and intro EDI in a support role and you're good to go in terms of characters. In terms of plot, cut Ilium entirely (I know... it sucks), focus on Omega and Tuchanka (Wrex returns plus Mordin's genophage drama). Cut Freedom's Progress, move Veetor to Horizon. Cut Haestrom, bring Tali in to Horizon to pick up Veetor after Shep and company have their initial confrontation with the Collectors. Give Tali the VS "Why, Shepard?" speech, but tone it down and she's convinced to come along on Normandy. ETA: Or, really, just use her speech from Freedom's Progress. Pretty much all the DLC will probably have to go?
One problem with cutting Ilium and the Shadow Broker: where does Liara come in?
I can be totally convinced to include more ME2 squaddies though, I love them all. (Although, Jacob...)