I really don't think Kaido has much to offer if he comes back. He was a fitting opponent for Luffy to finally prevail against and assert himself as a top-tier and Kaido proved his worth as World's Strongest Creature by taking on such a gauntlet and giving G5 a good fight. Passed Wano, he doesn't seem necessary. Especially since he would get up in Wano without Luffy and co around, would be kinda weird for him to immediately leave without causing trouble since that was his base of operations.
With Big Mom however, Oda could definitely still use her as an antagonist in Elbalf. She's a top-tier so giving Luffy a fitting fight works and she still has her reasons to head to Elbalf. Plus her falling in Wano is kinda disappointing, especially since she never faced Luffy when he was an Emperor or becoming one.
Ultimately, chances are they're dead but if not, Big Mom atleast has some potential as an antagonist for one last arc. Kaido seems pretty finished.