The 72-year-old won the Bundesliga, German Cup and UEFA Champions League treble with Bayern in 2013 before making way for Pep Guardiola and has not managed since.
He said he was weighing up the offer after meeting the club's president Uli Hoeness, chief executive Karl-Heinz Rummenigge and sporting director Hasan Salihadmidzic.
"Nothing is yet cut and dried," Heynckes told the Rheinische Post.
"I have to first analyse the whole situation. Four and a half years have gone by since I left Bayern and football has continued to change."
Heynckes, who has had three previous stints in charge of Bayern, including a brief caretaker spell in 2009 following the sacking of Jurgen Klinsmann, said the club hierarchy had offered him the role again "until summer 2018".
He said "many things were spoken about" during the meeting.
Bayern placed Willy Sagnol in interim charge after parting company with Carlo Ancelotti a week ago.
Former Borussia Dortmund coach Thomas Tuchel and Hoffenheim coach Julian Nagelsmann are viewed as the leading contenders to be the Italian's permanent successor.