Re-releasing old consoles as plug and play. But not so much the plug and play consoles, more so the fad of collectors who already have said consoles and probably the games as well, buying them up in mass, and people scalping the crap out of them like it's a must-have item. Yeah you know where this is going - the Nintendo minis are overrated collector's trinkets, IMO. The worst fad of all time? I dunno, but it's kind of annoying.
That's an example of a fad - something that people do because everyone else is doing it. A gimmick is something slightly different. That worst gimmick ever in gaming, were Tiger handheld electronic games, or Tiger wrist-watch games.