I'm a huge fan of cross breeding, but if you're going to cross breed to make changes to the breed just make a new breed of horse. Don't say that it's the same breed.
We no longer have the American horse, because other gaited breeds that it was used in developing turned out to be better and replaced it. If the breed will stand on it's own it will last, but there's nothing wrong with creating better breeds. Just make them as a new breed. Time will judge if they last.