What Daiba said in episode 274 was that he wanted to establish a "great empire of magic" on earth, not in Midland specifically. He was obviously referring to Ganishka's empire and to his role as the head of its magical efforts. A lot of things have happened since then, though. This ambition felt like it belonged firmly in the past when we last saw him in Berserk proper (fleeing Falconia with Rickert and the Bakiraka). Among other things, Fantasia made it obsolete.
Because the Continuation doesn't respect Berserk's established facts, it's possible they could retrofit his agenda so it would match what you're positing here, but even if they did, I would not expect it to play a major role in the story they're creating.
The World Tree is really a very different thing from Danan's cherry tree and it wouldn't make sense for one to turn into the other. Similarly, because Elfhelm was the tree and surrounding forest and they were completely destroyed (they didn't just vanish), it seems impossible that it could be rebuilt. The magicians lived in a village neighboring Elfhelm, but the "Capital of Elves" was the tree itself.
Anyway, I'm going to go straight to the point: Miura had a lot of things planned for the island, and that apparently included a window into the Skull Knight's past. The Continuation team decided to skip all of that. They probably didn't have a clue as to what he had planned, so they quickly got rid of the island. The magicians have been mute, powerless and barely even there since. That is not what Miura had set up for them, but sadly I don't think that will change going forward.