I will miss Streetpass. In Japan it might've gotten boring quickly since you'll get so many passes from so many random people, but where I live it's is so rare to get a random pass from a stranger. For 6 years I've only Streetpass'ed 10-20 strangers. I eventually met some of them by guessing who they were from their Mii's. We also held occasional meetups with people from Nintendo forums, SP was the main drive.
I hope they add this to Switch at some point after acknowledging it as a successor to 3DS as well. We almost always have 500x2 Mah spare battery thanks to Joycons. Why cant that be used to power such features while in sleep mode.