All the usual muzzleloader stuff, but some specific extra things for elk hunting.
1-Rope. Sometimes you get an elk down in a weird position, like on a steep hill, and the rope can help you hold the legs apart for gutting, or just hold the animal in place while you dress it. If you're going to leave the elk in the field for a while, you can hang up pieces, to keep the coyotes away, and to keep the meat clean and cool.
2-Good sharp knife and a sharpener. A large knife is better than a small one in my opinion. You may need to cut that elk up in the field, and your knife will get dull. It sure helps to touch up the edge while your working.
3-Some kind of folding saw. I have a folding hacksaw with a meat blade that I wouldn't be without. Makes field butchering so much faster and easier. Really helps for opening up that pelvic bone and ripping open the chest cavity.
4-A sturdy backpack. It doesn't even need a frame, just a strong and comfortable bag. One time I thought I had a good backpack, but the last elk I packed out tore it up so much, I got to camp nearly dragging the whole broken and torn mess on the ground.
5- Clean plastic bags. For hauling out the liver and heart, and for the rest of the meat if you need to pack it out. Bill