Not really a gimmick as much as it is a step in the right direction, but Sony really pushed Cross Buy on Vita. You got PS3, Vita and PS4 games all for the price of one game. There was times where you could make use of Cross Save. Inter connectivity between multiple devices is a massive interest of mine and now Microsoft are doing it and the Nintendo Switch is a portable and home console rolled into one. I think a portable home console is a dream gimmick, but obviously held back in some regards to technical limitations.
When it comes to gimmicks I also adore the share button.
I'd say not really since its now standard on all three console controllers. I'm talking moreso about features and gimmicks unique to one specific platform, that are either not utilized as much as they could've been, or are a bit too impractical and under developed to be used often. In which case, the PS4 Touchpad and Speaker are better examples.