User blog comment:B214/High School DxD EX/@comment-50.139.232.230-20160312041646/@comment-93.105.246.116-20160312144941

Well he probably had some of the kids with Kunou and Kiyome Abe. Could also be Tsubasa Yura.

Another candidate is Mira assuming Riser would allow it. Grayfia could be another too - since Sirzechs is implied to be dead in that future.

Possibly also Yasaka, Gabriel and Serafall but those are just assumptions without proof. Well Yasaka did flirt with Issei a bit.

And one more thing is he may have more than 1 child with some of the girls we do know he shagged. Like he may have a 2nd child with Rias but they would brobably be still kids if not babies at that time.