I'm slowly replacing the mim parts in my 3" 686-6 along with doing some other modifications.
I understand many believe that the modern mim parts are fine but it makes me feel good and I enjoy personalizing/"enhancing" my guns.
I already installed an Apex Evo IV DAO hammer which (I was pleased to see) was a drop in replacement along with their extended firing pin and firing pin spring. I also installed an Original Precision lock plug, D&L sports fixed rear sight, and a Wilson Combat mainspring / 14# rebound spring.
Other parts that I've purchased but haven't had time to install include a Power Custom trigger, NOS stainless steel rebound slide (which I also polished), NOS stainless steel bolt, Apex tool steel cylinder stop, and an SDM Fabricating thumb latch.
I'm enjoying reading through Jerry Kuhnhausen's book, watching Brownells and other instructional videos, and plan to take my time to ensure proper fit and function.
Again, probably not necessary but more of a hobby and I like to believe that I'll have a personally "optimized" 686 when I'm finished.