For a movie I think it'd need to be an original story to provide some creative freedom.
The film could only be a small slice of the larger universe so roll with that. Keep it personal.
The Great Houses are pretty iconic, so the immediate setting would benefit from being somewhere they can meet. I'd recommend Solaris City, late 3040s. This leaves the option of bringing the Clans in for a sequel.
Make your protagonists a mismatched pair of Mechwarriors - one AFFC, on DCMS; one seasoned Solaris veteran, one new arrival; one who starts with a 'Mech, the other already Dispossessed - and play them off against each other when they have a mutual enemy. One of them loses his/her 'Mech, the other loses his/her significant other and the only way for them to get them back is to take on the 'boss' of Cathay or Montenegro. After some 80s style action movie heroics and 2-3 small 'Mech battles they emerge having rescued the significant other, recovered the 'Mech and they've maybe got another love interest and another 'Mech to balance things out. Maybe they form their own stable, maybe they decide to catch a ride to Outreach and go merc'...
