“You don’t know what you want do.”
That’s not a big issue in and of itself, undecided or undeclared is one of the most popular majors in freshman applications. And colleges know that a significant percentage of students change majors, and many of them actually recommend taking the gen ed requirements so you get a sense of what you might want to major in down the road.
“Now I’m wondering, is it is wise to just tell the WL that I really want to go to their college, and would be willing to change my major to just Undecided?”
I wouldn’t do that, as others have also suggested, in some colleges, undeclared is harder to get into than some majors. Here’s the thing, most of the colleges on the waitlist have yields from 25-30%, which is low given their selectivity. I think you may have a good chance that you’ll get one of them.