In theory, Squaresoft could make another Xenogears game, or Bandai Namco could make another Xenosaga game, even without any input or approval from Tetsuya Takahashi. They own the intellectual properties, which is why he keeps starting over. The major issue here is that Tetsuya Takahashi wants to keep doing the same series, but he doesn't have the sense to demand ownership of his IPs as part of the contract when he creates them. If he could wrangle personal ownership of a property he made, he'd be able to keep working on actual sequels instead of spiritual successors no matter what company he worked for.