If you want my opinion on BWD as someone who, for the record, hasn't played that many Kirby games: I think he's the clear fourth choice. It's funny, because once you get down that far a lot of franchises "open up" with a lot of characters you could potentially choose, but... yeah, he's just so obviously the next most important character after the three we already have.
Prior to Ultimate's release, I was pretty against including one-off characters from series with recurring characters - meaning the likes of Geno, Sheik, Midna... Even the likes of Skull Kid or, indeed, most of the alternative Kirby characters people are mentioning rubbed me the wrong way because they were basically one-offs with like two extra cameos.
I've lightened up on that over the years - sorta - but... yeah, I don't really see any other Kirby characters getting in before BWD.