User blog comment:B214/High School DxD DX.4 discussion/@comment-24421137-20170720035349/@comment-10330822-20170721220246

They seem to be going in birth order of the children, aside from Rias obviously, since Akeno and Asia's children are the eldest. If they stay that course, Rossweisse and Kuroka/Koneko are next. Since Koneko and Kuroka had children, and were probably impregnated, at the same, odds are Issei will propose to both at the same time as well. Part of me suspects Issei will propose to Rose after the battle with Typhon, and Kuroka/Koneko after the Vali Team's battle with Rias's team.