As we know, R.L. Stine said on Twitter that Slappy was coming back, but some people don't believe him, oh well, but I found this article online.
The article says he will return to play Slappy, however, this article doesn't confirm it 100%, for I'm not sure were the author got this information, but still, it could be legit info, we're just going to have to wait and see.
As for me, I strongly believe this, because a Goosebumps movie without Slappy would be like summer without ice cream, but let's see what news says.