Warren Leight’s comments about the 500th episode indicate that it wouldn’t necessarily be out of the question for characters from other shows to turn up, so Christopher Meloni could take a break from Organized Crime’s intensity to drop by SVU.
And there will technically be characters from a grand total of eight other shared universe shows that could turn up, thanks to One Chicago and FBI/Most Wanted/International (hopefully with one memorable character returning) on top of the Law & Order spinoffs, but I’m guessing the closest thing to a sure thing would be Meloni as Stabler.