I unfortunately don't think they'll reshuffle the base roster in any way, or merge any existing characters into a single slot. Like with Mario Kart 8 Wii U I think they'll simply decide to have a row that is easily identifiable as "the DLC row"; considering not everyone who owns the game will have this content, I don't think they'll want to randomly mix them into the selection in the ways being suggested above.
I've been arguing this elsewhere already, but I think Birdo works quite well as the "teaser" character, on multiple levels. She was the very first new character developed for Tour after the 8DX cast was ported over (showing she has had priority in the past); coming with alternate colours will likely be positioned as making up for the lack of other characters this wave (and is pretty reminicent of how the Yoshi and Shy Guy colors were treated in the vanilla game's DLC); and of course, she fits the themeing of the Yoshi's Island course.
One row means six more in the remaining two waves, three each; which if you want to bring up similarities to vanilla 8's DLC is also the same setup as that game (why I also anticipate a set of kart parts showing up eventually). Of those six I think you can easily lock in Diddy, Funky, Petey and Pauline; but I could see a variety of combinations for the remaining two. With how Wii-heavy the course selection is the former two will definitely be priorities though.
I really wish it were possible to bring over the full Tour roster. There are so many characters in the game already, and more likely coming to it in the future, so it'd be disappointing to not see the majority of them come over. DK Jr., Birdo, Diddy, Paratroopa, Petey, Funky, Wiggler, Honey Queen; every veteran sans R.O.B. is either already in or has signs of being on the way. Then we have all the newcomers we've seen so far: Pauline, Peachette, Hammer Bro, Monty Mole, Dixie Kong, Captain Toad, Nabbit, King Bob-omb, Kamek, Chargin' Chuck; and some that could be on the way (different levels of probability for these, from most to least plausible based on evidence): Luma, Piranha Plant, Boo, Baby DK, Baby Wario, Goomba, Dolphin and (Gooper?) Blooper. Would be nice to try and fit them all in, and some of the more stand-out costumes as well, but I just expect them to limit things to 7.