Borussia Dortmund are known for an exciting style of play, but since the Bundesliga winter break, they've been on another level offensively.
Kalou joined Hertha from Lille in 2014 and his current deal had been due to expire at the end of the season.
Alonso, 35, announced on Friday that he would retire at the end of the season.
Arsenal's Hector Bellerin returning to Barcelona and Bayern Munich's Kingsley Coman joining Manchester City are among Wednesday's emerging whispers.
Gotze, 24, was diagnosed with a metabolic disorder last month and withdrawn from training.
Castro, 29, joined BVB from Bayer Leverkusen in 2015 when the club triggered an €11 million release clause to sign him.