While it's still early in the summer, there have been quite a few transfers already completed that will be ones to look out for next season. Let's take a look at some of the transfers that could end up having big impacts on next season.
Corentin Tolisso to Bayern Munich, 41.5 million
While Tolisso may not be a "proven" European star yet, he will most likely be a key player for Bayern in the 2017-18 season. With Xabi Alonso retiring, Bayern will need a new center midfielder to take on Alonso's minutes. So far, Bayern have only signed Tolisso and Rudy as center midfielders, and neither of the two truly replace Alonso.
It remains to be seen who will end up being Alonso's replacement. Joshua Kimmich had another good showing at the Confederations Cup, and his ceiling seems to only be rising and rising. Kimmich is probably the most likely player to fill that spot in the Bayern midfield next season, but in addition to the new signings Tolisso and Rudy, a certain Renato Sanches will also be vying for the spot.
Omer Toprak to Borussia Dortmund, 12 million
This should be actually a great signing for Dortmund. One of their biggest problems this season was depth at the back, and signing a proven center back like Toprak should be a big help, even if they did sell Matthias Ginter to Monchengladbach. Their lineup should look something like the below next season:
I think that Toprak signing does a lot to shore up the defense -- Toprak is a big improvement over Ginter. Additionally, the Dahoud signing looks promising -- while he had a subpar season for Gladbach, everyone knows the type of potential that Dahoud has. Dortmund has incredible depth this year, and should be a team to look out for (as they always are).
John Anthony Brooks to Wolfsburg, 17 million
Brooks has established himself as one of the premier young center backs in Germany. He has had a few great seasons for Hertha, being the marshal of their backline. Brooks has also proven himself on the international stage, having an excellent Copa America for the United States last summer.
While he may still be young and may make a good number of mistakes, he has enormous potential and could evolve into a world class player. One has to make comparisons to Jerome Boateng, who also stands tall at 6'4" and similarly started his career at Hertha Berlin. When Boateng was younger, he had a reputation for being mistake prone; Mario Balotelli's goal against Germany in Euro 2012 comes to mind, where Boateng fell asleep on defense.
Wolfsburg may have just barely avoided relegation this season, but all this money thrown at new signings has to pay off at some point, right? Wolfsburg let go of two big stars of theirs in Rodriguez and Gustavo, but in my opinion that squad needed a clearout. I think Wolfsburg are trying to reinvent themselves as a club, and all the more respect to them for it.
Alexandru Maxim, Viktor Fischer, and Rene Adler to Mainz for a combined 6 million
This one is obviously not just one transfer, but the three person combo of Maxim, Fischer and Adler promises to work out well for Mainz. Mainz are famous for their ability to sign good players on the cheap, and this summer seems no different.
While Mainz did offload their star striker Jhon Cordoba to Koln for a whopping 17 million Euros, I think their squad is overall looking better than it did last season. Adler is a proven Bundesliga goalkeeper who still has it in his locker to be great for a team like Mainz, while Maxim was not too long ago one of the breakout stars in the Bundesliga. Viktor Fischer's stock may have plummeted after a mediocre last few seasons, but for 3 million it's worth taking a chance on him. A team like Mainz could be perfect for him to rejuvenate his career.
Leverkusen let go of Calhanoglu and Toprak for a combined 34 million
While I already covered Toprak from the Dortmund side of things, I think it is also important to look at Leverkusen's side of this transfer window. After a seriously embarrassing 12th place finish for Leverkusen last season, something needed to change. They had already gotten rid of Roger Schmidt, their manager of the past several seasons. The sales of Calhanoglu and Toprak just reinforce this shakeup.
They haven't really made any star signings yet this summer, but stay alert as Leverkusen will probably be bringing in at least one big player. Also keep an eye on the Chicharito situation as he had a very disappointing season for Leverkusen last season and still holds lots of transfer value.