I've said this before, even before SM09 aired, but Ilima is one of the few Trial Captains who could be omitted from the anime without drastically affecting his Trial, as his Trial could easily be taken over by Hala. And that's exactly what happened--Hala played Ilima's role. In that case, it worked. Professor Kukui took Ash to Hala himself to start Ash's Island Challenge (which sort of mirrors the games), so it made sense that Hala might want to preside over Ash's first Trial himself, especially after he learned that Tapu Koko had taken an interest in Ash.
Lana, Kiawe, and Mallow play larger roles in their Trials in the games, so it would be a little harder to completely omit them--or, at least, omit their roles. At the very least, they would probably have to be replaced. Olivia could take on their roles herself, but it would be rather difficult for her to run all three Trials and the Grand Trial by herself. It's possible their roles will be taken by OCs, or the writers work Lana, Kiawe, and Mallow into their game Trials, but in different ways. For instance, perhaps Mallow accompanies Ash to the Lush Jungle Trial, and she comes up with the idea to cook a meal to draw out Totem Lurantis. Perhaps Lana accompanies Ash to the Brooklet Hill Trial, and she notices the ripples in the water where the Wishiwashi are gathering.