I was going to say FA or FE.....probably FE most of all, but I agree you will probably be keeping the FM for awhile. Lovely camera. Personally, I would like the built in metering of the FE. I do agree with you that when I haul around the old F4S I have, it can become a heavy load....sure does look cool though.....especially with the old Metz potato masher flash!! Makes you look like you know what your doing.
My favorite light weigh solution is an N75. You can use your old lenses or any of the AF lenses.......and it is feather light. If you have a bunch of AF lenses, I recommend you look into the N75......I believe you can still pick up good used one for next to nothing....I got both of my for around $50 per copy a couple of years ago. Certainly not as heavy duty as the F4 or your FM, but wonderful nice little consumer grade camera.