From what we already know would appear in Season 7 at some capacity, here's who I think who else would make an appearance.
- Chen (flashback)
- Clouse (flashback)
- The unnamed Earth Elemental Master
- Garmadon (flashback)
- Morro's old classmates
- Samukai (flashback)
- The Great Devourer (flashback)
That's all I could think of. Who do you have in mind?
Edit: Forgot to add the Devourer.