The above accessories is very detailed and a good reference.
I would also add:
-pair of small needle nose multi-pliers (e.g. Leatherman)
-multi-allen wrench set
-Multi-Screwdriver Set
-Brass Bore Guide for range rods (protects the muzzle)
-Speedloaders for your powder type (Loose or Pellet)
-Collapsible range rod (Like from XS Sight Systems or CVA)
-Main Box for all off the stuff (like a larger plastic tackle or tool box) (I leave this in the vehicle)
-A shoulder sling hunting pouch for your essentials (I carry most of the following depending on how far from the "Main Box" I am)= Small plastic box to house loose items listed), nipple/breach plug tool or wrench, Charged loaders, primers/caps in a loading tool, nipple pick, (small ziplock bags with about 30 dry, 10 alcohol, 10 lubed patches), Collapsible range rod).

Funny thing about muzzleloading... all of the gadgets and gizmo's that you end up with.
I'm still adding nit noids.

My .002