Maybe if that's the case then the choice is for him to stay at Leverkusen for at least one more season, he can use that to prove that he can indeed handle a bigger club than Leverkusen. The reason is that currently I have also heard a lot from those who say that Xabi might just be lucky this season, therefore he must be able to prove himself by staying at Leverkusen and showing the same thing as this season again next season. However, I consider these words to be normal because Xabi has not yet fully proven that he is a consistent coach.
Real Madrid currently still feels at home with Ancelotti, it seems they want Ancelotti to continue the regeneration process first before they appoint another coach. The possibility that Zidane will return to Real Madrid or not is still a big question, because up to now he has not yet spoken about his coaching career after deciding to take a break some time ago.
I think it was a good decision and staying one more season at Leverkusen and being able to bring Leverkusen to consecutive championships will make his name even better in the coaching arena. Person always attribute luck to Xabi Alonso's performance at Leverkusen, even though this happened inseparable from his hard work with the entire team and players. That's why he needs to prove it next season so that people can see that he does have the ability to organize strategy.
If Ancelotti still intends to stay for the next season then I am sure Real Madrid will not look for a replacement. Zidane's name is very familiar there and he has brought several prestigious titles to Real Madrid, but as you said, maybe not for next season because Ancelotti has not yet decided to leave Real Madrid.