Being that SVU is pretty resolutely a cop procedural, I'm not actually sure what its previous showrunners did besides keep track of headlines to rip, but Leight was the first one to really introduce any sort of extended arc across his seasons. (Previous to S13, the only "arc" I can really think of was in S8, but it was mostly "Mariska is pregnant so we need her to be 'undercover in Oregon' for a while.")
I don't know that these mytharcs always worked, haha, but they were incredibly important to SVU reinventing itself as a million-season running show, and really thinking about what the cop procedural drama can do.
Leight's seasons (S13/14/15/16/17) made the NYPD of the Law and Order universe into a kaleidoscope turned on bureaucratic corruption, police failure, the intensity of a system on the brink of both success and collapse in NYC. Wedded to its times (in both good and bad ways--some ripped from the headlines episodes were really ripped...a bit too soon), Leight's run on SVU did not shy away from taking a holistic look at the entirety of policing in NYC and creating a fictional alter ego poised to engage its complexities. Thanks, bruh!
I'm writing this, of course, not because I have any particularly strong investment in who's running or writing SVU (like, it's a cop procedural. XD If they keep with Leight's mytharc formula, power to 'em; if they don't, then it's not like SVU didn't do that for 12 years previously).
I do, however, have a large investment in who runs and writes SPN.
Jeremy Carver will be stepping down as showrunner after helming us for S8/9/10/11 (and writing for S3/4/5), and writers Robbie Thompson (S7/8/9/10/11) and Jenny Klein (who's been working on SPN in various capacities since S4) will be moving to other projects as well. WHICH MAKES ME A LITTLE NERVOUS, not because any one person is leaving, but because they're all leaving at the same time--that's a lot of changeover! And because I've really enjoyed Carver as a showrunner. It takes cojones to deal with us, man. XP
Whatever S12 brings, there's going to be lots of new chum in the water. I hope we're nice to them, and that they're nice to us!