European governments' plan for private-sector creditors to help Greece's next bailout without triggering a default were thrown into doubt Wednesday, as senior German officials resurrected a once-rejected proposal that would cost investors more.

The German proposal—calling for investors to be encouraged to swap Greek government bonds for new bonds—had been ditched a month ago after strong opposition from the European Central Bank and governments including France, because it would lead to Greece being called in default by rating...