I think that within limits any horse can do any job but you have to be realistic. For example, an Earth type draft horse is not a good candidate for grand prix jumping but they can hop over a 2 foot fence just fine. A racing bred Wood TB is not going to be great at cutting cows but he can learn to pay enough attention to rider cues to turn a cow.
The challenge comes when your favorite horse does not fit the job you want to do. No problem, you can help your horse learn the skills and perform to the best of his ability and love him for the effort not the results.
Many times the skills a horse learns to do a job that is not natural will help improve his overall usefulness and make him a safer more rounded individual. An example would be my half TB, Fire mare that I am teaching to barrel race. Cerise does not naturally like to work off her hind end or flex laterally. She much prefers things that involve going fast in a straight line. The extra time I have spent working with her on flexibility and collection have improved her overall condition and made her a better trained horse. I have had to do some special nutritional support for her to help her cope at times with the extra physical and emotional work. I don’t expect her to be super competitive but she is happy to be getting the attention and I am getting to do what I enjoy. Madalyn