Becoming the new manager of a team the manager is expected to perform better than the former manager so Ten Hag will be expecting to outperform Xabi Alonso by winning the champions League, the league title and league cup.
Indeed. A new coach/manager should be expected to improve the team and will give a better achievement. Since Xabi Alonso failed to win the Bundesliga title again, Ten Hag is expected to win it in the next season. I'm sure UCL trophy isn't the main target, domestic trophies will be the main target. If Ten Hag can win the domestic trophies, he will likely be considered successful in the next season.
Last season Leverkusen couldn't maintain their decent performance and they didn't give much competition against Bayern Munich Ten Hag needs to fix things so that his team can compete with Bayern Munich next season.
There was a big decline with Leverkusen performance last season. It is not bad that Leverkusen changes their coach. It is expected there will be some changes in Leverkusen tactics. If a new coach can do it well, we can expect there will be an improvement in Leverkusen performance. Ten Hag is one of top coaches in the world, I think he has the ability to return the best performance of Leverkusen.