There are circle pits, and then there are "System Of A Down playing the end of 'Toxicity' at an arena show" circle pits. The band took to Twitter to showcase pretty much the entire floor of a stadium losing their shit completely; so much so that it kinda looks like several ant colonies going in circles. So there you have it – an aspiration for the size of mosh pit your band can induce.
System Of A Down is currently slated to play the Sick New World festival alongside their nü-metal peers like Korn and Deftones.
As for any new material, don't hold your breath. System Of A Down vocalist Serj Tankian has been seen the holdup for a new album for years now, though he appeared to come around on the idea in recent years saying "we'll see what happens" and just kinda shrugging at the whole thing. Still, drummer John Dolmayan and major advocate for a new album has made it abundantly clear that he's pissed there's no new music… and even said maybe Tankian should've quit the band years ago.