What parts of this game could work with the base game? Characters? Items?
Yes to characters, yes to items. I mix in both.
In theory you could mix in the green and yellow encounters though I wouldn't recommend it.
What expansions could be used with Island of dread (in any)?
The class decks.
The 2 item decks.
Maybe the green and yellow encounters from the encounter decks, but I wouldn't recommend it.