unfinished business

noun

something that a person needs to deal with or work on : something that has not yet been done, dealt with, or completed

A matter that still has to be completed or dealt with, sometimes with an ominous or threatening implication.
“He gives the impression that it took time for him to come to terms with his feelings towards her, admitting that, “I'm sure we had some unfinished business”, […]”
“The measures for rail that were genuinely new were welcomed, although some had hoped for more. Many of these tied up loose ends around the network - unfinished business left hanging by the last government.”
A matter that still has to be completed or dealt with, sometimes with an ominous or threatening implication.