Non of the candidates is really of Mario's talent, though Ajax Amsterdam's Christian Eriksen is close. Jackson Martinez is also excellent, but doesn't seem to fit as good in Dortmund's strategy to develop young talents.
Chealsea's talent Kevin de Bruyne had a promising start but failed to develop good in the recent years. Probably he also regrets his move to Werder Bremen. Son, playing for Hamburger SV, didn't have a convincing Goalimpact so far despite good critics. He certainly has talent, but it would be a bet if he really can do use of it better than in the past. Bernand is supposed to be a Brazilian supertalent. He currently plays at Atletico Mineiro. There are not too many observations of him in my database, but he managed to be above his team average already. A risky choice, but it may work out very good.
Bottom line: Eriksen seems to be the best option.
If you see other rumors, please leave me a link in the comments and I can add further players.
Here are some more rumored players as hinted in the comments.
Kevin Volland had too little Goalimpact so far to be recommended. Dortmund would need to invest in his talents first until he will be effective. Christian Benteke showed his talents more than once in his career, but experienced ups and downs already in his young career. Maybe he suffered from frequent team changes. Thiago is excellent. According to Goalimpact certainly a player that can turn out as valuable as Götze. Dzeko is different he started getting high ratings in his second year at Manchester and could in the current form reinforce any team in the world. A good option if a complete player rather than a talent is thought after.
Internal solutions could be Bittencourt and Hofmann. The former seems not yet convincing although he lately played some games for the A team. Hofmann looked more promising, but saw his measured performance dropping in the last year.