The gifted 25-year-old, who has been at Dortmund since 2011, had hinted at leaving last year with several clubs in Italy and England's Premier League interested, before signing a new two-year deal in July.
"No decision has been taken yet and of course everything has to be thought through because the length of one's career is limited," Guendogan told Monday's Bild newspaper.
"To be honest everything is still open," the creative midfielder said, adding his next contract would be longer. "I think it will go into that direction. When I sign, it will most likely not be for just a year."
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Guendogan, a 2012 domestic double winner and Champions League finalist in 2013, has earned 16 international caps but missed Germany's 2014 World Cup victory in Brazil when he was sidelined for a year with a back injury.
Dortmund, currently second in the Bundesliga, will only receive a transfer fee if they sell the player at the end of this season with Guendogan able to leave as a free agent in 2017.
(Reporting by Karolos Grohmann; Editing by John O'Brien)
