Here is a
seasoning mix I use for smoking baby back ribs -- it should work fine for a roast as well. My wife was concerned that the chili powder would make it spicy, but the long cooking seems to defuse that.
Another idea is the
Syrian shish-kebab seasoning I learned from a co-worker many moons ago. While the recipe is for lamb, it works great with pork tenderloin, and I adapted it for stew beef as well. Last night I used a version of it (added granulated garlic) for pork tenderloin scaloppini, and would consider it for a pork roast. I suggest marinating at least 24 hours.
A different co-worker slow cooked a pork butt with a "stuffing". He finely chopped green bell pepper, onion, garlic, and green olives, seasoning with salt & pepper. Using a long, thin knife he cut deep slits in the meat and stuffed in the stuffing (in small quantities), then slow cooked the butt for 8 hours. It was amazing!