I'd continue using the FM2n. But, maybe add an FE2 (or the aforementioned FG/N2000 series cameras for when you want auto modes and TTL flash). That said, F4's are pretty cheap nowadays. Could get a pro-grade camera that has most, if not all of the features you want, plus some added features that are nice.

My main complaint about the FA, is not all the features, but the F3-style meter display. It's not that informative in manual mode. You're either overexposing by an unknown amount, at correct exposure, or underexposing by an unknown amount. The F4's meter display (introduced on the N8008) is much better. Is a bar-graph style display that goes from +2 to -2. Very easy to compensate for scenes that aren't middle-toned.