Fallon, Wiig, and Fey have come back a few times each, Hader, Samberg, Ferrell, Poehler, Silverman was on the show for I think 1 year so she doesn't quite count. And that's really it the past few years. Over 22 episodes a season, it only ends up being a few a year, which isn't too bad. They definitely pop back up pretty often as a guest appearance for a sketch.
A lot of times they're either doing the regular promotion for a movie or TV show or it's the premiere/finale episode so you bring in a fan favorite/old cast member.
The show's biggest issue is just the writing at this point. They've had capable actors, but even with their strong cast members, the sketches are just flat and repetitive. The cold open is always the same political sketch and it's basically "Look, we paid attention to what's happening this week". Too many sketches just kind of trail off and end... you can tell if they didn't have the applause meter, the audience wouldn't laugh or know it was over. Sketches don't have a payoff or twist at the end, it's just the same joke, repeated 3-4 times with a different set-up.
You don't need big names or future stars to have a funny show... if anything, that hurts sometimes. We all knew Wiig was the strongest female they had in years, we all know she'd be in movies and eventually move on, and we had to deal with 4 recurring sketches a week with her front and center. So when they have that strong star, they can lean on them too much rather than think of new or more clever sketches. I love Wiig... but if I had to watch Gilly one more time, I would have lost my mind
Target Lady too