If we do get an "Original retrospective" Anniversary house, Swamp Yeti's are a must. I'd say they were the most iconic original house monster of the past 5 years. Doesn't hurt that they appeared twice (and stole the show both times imo).
My top picks for original roles from each year, at least for the purposes of an anniversary house. (Not including Yeti's for either 28 or 29)
26: Chance, maybe a cameo from Amut from TotA.
27: Scarecrows are another definite. Maybe Bones as well?
28: I think the Wicked Witch and/or a Clicker from Seeds would be really cool to see come back.
29: Maybe the Grinner or Punk from Graveyard and/or a New Nightingale? I know one thing for sure though, you're not gonna see any Mouthbrooders this year. Pretty sure those costumes were scarier for the actor than it ever was for the guests.