I was reading an article on missing supporting characters and I wondered if there's any way to put the pleasure of returning old characters into a campaign.
If it's a returning campaign, sure. But how do you put a "returning" character into something that hasn't ever existed before?
Well, you need player buy-in, I think. Without player buy-in, you have to drop hints about the character and have them talked about for sessions, before they ever show up. Maybe you can get that little frisson.
A way you might do it is having players generate a background and plucking from that. (The character probably has to have good associations rather than bad.) Obviously, it's easier if your players do lists with character names, but it's not always necessary. If your player had a character background that mentioned a childhood as a military brat, then you can invent a character, one of whose attributes is "Former friend of player character" and their lifestyles explain why you haven't seen the character before.
I wouldn't do this sort of thing for every player, and certainly not all at once...then it becomes, "Hey, what plot requires all of these people from our past to show up, and what villain might be doing it?" You don't want that.