Next season will not be easy for Leverkusen if they don't give Xabi the new players he wants, we know they will be competing in the Champions League next season and there will be many teams they will meet with fanatical squads, in this case I hope Leverkusen management spends money to convince several players to strengthen Leverkusen next season.
I don’t think Bayern Leverkusen needs a lot more new players if their current players are not leaving to join other teams now. He will only need few players to join to the team which I am sure will not be a big deal or a difficult one for the club to provide the finances for it. He can still build on his existing players to make them a formidable team as they were in this season. I have full faith in Xabi Alonso and I know he can’t do it again next season.
Of course, with their success this season we cannot be sure what next season will be like, of course the Bundesliga clubs will also strengthen their sectors so they can compete with Leverkusen and other teams, next season will of course be different from this season.
I won’t see a major change in the Bundesliga next season if they still wants to play with the tactics they use this season. They can as well have a plan B in case the existing plan doesn’t work in their favour since many teams would have already understood that from them now. Asides that, I don’t see any major change that can have effect on their performance come next season.
There will definitely be additions or changes to players. It seems there are players who are out and Xabi is also definitely developing a strategy to target tag players. Yes, however, now Leverkusen has become a top club. and they will join UCL next season. So, in any case, Xabi has brought together several more talented players who will be his key players in the future. But as you say, you don't need many, just a few who are really suitable and can make the team much stronger.