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.
Of course next season may not be this favourable for them if they don't step up there game however just like you mentioned champions league is not an easy league were a club will go without getting prepared because it lies all the strongest team from the various local leagues, actually we know that Bayern Leverkusen has been the best club this season on German league but there performance this season will not guarantee there success next season because after the performance of all the clubs they will fall back and calculate on how they can tighten there club on those areas they feel are not doing well so that when they come back next season it will be a game changer for them.
So actually the only way for Bayern Leverkusen is to get more good quality players that has a good champions League experience because there would be a serious competition next season and also I even doubt if Bayern Leverkusen will be able to win the German league again next season because Bayern Munich and Borussia Dortmund will be serious competitors against Bayern Leverkusen.