During the first season of That '90s Show, a few of the cast of the original That '70s Show made their appearance, mainly to introduce and pave the way for their children to take the stage and carry the show forward. This included both Ashton Kutcher and Mila Kunis, who play the couple of Michael Kelso and Jackie Burkhart, whose son Jay is one of the core figures in the Netflix series.
So, considering the pair are present and have made an appearance in That '90s Show, can we expect them to show up again in the second season? Simply put, no. Kunis has spoken with Entertainment Tonight, where she specifically revealed that neither her nor her real life husband of Kutcher will be popping up for a cameo.
"No. I mean, we did our thing and they introduced our son in the show."
<social>https://www.youtube.com/watch?v=D9uKCRTWGUA</social>
This doesn't necessarily mean we won't see Topher Grace, Laura Prepon, Wilmer Valderrama, or other returning stars (each of whom had more significant roles in That '90s Show anyhow), but it does probably affirm that the second season will have even a greater focus on the new young cast.