So I'm kind of amazed that Noctis was able to just walk around on his own without guards and whatnot in the anime. Lets say that Noctis and Prompto are kidnapped/taken hostage on one of those days out (maybe walking home from school or something similar to the anime). The kidnappers intent was to get Noctis, but poor Prompto is taken along as well. Up to the filler if Noctis is being held for ransom, a group that dislikes the royal family or whatever. But they decide they're bored and want to play with Prompto while poor Noctis is forced to watch.

+if there is H/C afterwards, but can be hurt and no comfort
++if the kidnappers mention how pretty Prompto is
+++for Prompto believing it's better that it happens to him than Noctis
++++can be just touching to all the way to attempted noncon or noncon.