Why was it that you think Neville quit his career as an Auror? Do you think he never really intended to be an Auror forever and took the job just until the Ministry was sorted out. Do you think in the event of a great emergancy, he would be invited back to serve under the Auror Office once again.
J.K. told us the Neville and Hannah got married, but she never mentioned any children between them, the way she did with Luna and Rolf. Do think they ever did have any children, or that they were just content to be one of those childless couples. With all the children running through the school, they might already feel like they have one of the largest families in England.
Or would they want to add to that large family with some miniture Nevilles and Hannahs?