Conformation is only part of it. If you horse loves to cut cows and is good at it, you're not going to chalk it up to his conformation. Point in case: Seabiscuit. He was stocky and chunky, but ended up being extremely fast.
It more matters between what was said above: What you like to do and what your horse likes to do. If your horse trusts you enough, it'll do what you ask of it.
Maybe experiment a couple of different disciplines? For example: I contacted a previous owner of my mare's sire. She said they kept one of his other foals, a gelding. He was originally trained for barrels but just didn't put his heart into it. By chance, they happened to use him at a friend's place for roping. He took to it immediately and ended up being his niche.