I imagine four scenarios:
1 - The girl is MC's daughter, so story about reconnecting with the daughter and get together again with his ex-girlfriend
2 -The girl is not MC's daughter, so story about him becoming her stepfather and get together again with his ex-girlfriend
3 - Ex-girlfriend has no interest on MC romantically and her daughter (not related to MC) become the love interest after some time skip
4 - Oyakodon