Can only relate what I use:
Filters, lens hoods, light meter, tripod and cable release on occasion, discrete bag, *film*, and yes, a soft release. I used softies on my SLRs back when they were my primary gear, now I have a softie on my M cameras, but not on any of my other rangefinders. Seems to be M specific for now. That's about it for needed accessories.
Oh, one more. When out shooting with barnacks, I keep a small folding pair of scissors in a film cannister, in my bag. I trim leaders on a "need to trim" basis.