There’s an AA no one has mentioned yet we might get to see.
Off the top of my head, iconic magical creatures seen in the dark forest, and what I believe to be the likelihood of seeing them on the attraction:
- Centaurs (very likely)
- Unicorns (likely)
- Spiders (maybe in some form)
- Aragog / Acromantulas (probably not; thanks FJ)
- Hippogriff (probably not; thanks FotH)
- Werewolf Lupin (possibly...?)
- Dementors (probably not; thanks FJ)
- Dragons (probably not; thanks FJ)
- Grawp / half-giant (likely)
- Thestrals (likely)
- Ghosts (hmm... nah)
Other iconic non-creatures...
- Voldemort (highly unlikely - going back to the first year would be extra loopy even if we don't have a timeline)
- Hagrid and Fang (maybe? I wouldn't mind it, even if Hagrid is already featured, though not seen, in FotH)
- Ford Anglia (very likely)
- Whomping WIllow (is "by" the forest... hmm... probably not; thanks FJ)
- Death Eaters (probably not)
Can't think of anything else right now. Of course there's a few more magical creatures to be found in the Black Lake, but I don't think it will be featured.