The secondary villains should be added to the cast if they decide to make a sequel, but, no, they should not have replaced the choices they made, which were awesome.
And, yes, Seymour sucks. Badly.
Doesn't matter. Yuna was the real hero(ine) of FFX and much, much cooler than Tidus. Imagine how much more playable she'd be. (Alternate costume gunner, amirite). Also, another girl on the side of Cosmos would have helped.