Borussia Dortmund have made their first signing after Jurgen Klopp's departure by landing midfielder Gonzalo Castro.
Liverpool give U21s more game time than Premier League rivals but top flight lags behind Europe.
Eintracht Frankfurt ended the season with a 2-1 win over Bayer Leverkusen.
Stoke have signed defender Philipp Wollscheid on a permanent deal following his successful loan spell with the club.
Bayer Leverkusen manager Roger Schmidt has signed a three-year contract extension until 2019.
Bayer Leverkusen all but ended Hoffenheim's hopes of qualifying for Europe next season with a 2-0 win at the BayArena.