Gladio can get pregnant, and during that final night before they face Ardyn, him and Noctis share a moment of passion (just the two of them or Ignis and Prompto can join in as well). After everything is over and Noctis is gone, Gladio finds out he's pregnant.

I would love if Ignis and Prompto are there for him and they decide to raise the child together.

(Because the thought of pregnant Gladio has planted itself in my head and won't go away.)