Alastor Moody a.k.a. Mad-Eye
Alastor Moody from Harry Potter has been through a lot and seen a lot. And he would definitely have some interesting tales to tell. He's not a very trusting sort. Not that you can blame him. But he would be the kind a guy to bring his own food and drink in a tightly sealed container. And you would probably catch him out of the corner of your eye taking sips of something peculiar in a flask.
This is the Mr. Darcy before he falls in love with Elizabeth Bennett. The Darcy at the beginning of Pride and Prejudice is a bit of an ass. I can imagine him sitting in a corner brooding. You try to engage him in a conversation and it would go like this:
Me. How's the turkey, Mr. Darcy?
Darcy: It's tolerable, but not handsome enough to tempt me. You had better return to your dinner and enjoy it, for you are wasting your time with me.
Captain Ahab from Moby Dick would by so annoying to have dinner with. Can you imagine. All he does is talks about that damn whale and how awful his life has been. What a downer! Somebody please put a harpoon in that conversation!
Gollum from The Hobbit and Lord of the Rings is probably at the top of my dysfunctional list. He would probably be upset because you 'overcook' the turkey. After all he likes it raw and wiggling. Also he would probably argue with himself over who gets the wishbone. He'd also probably try to eat the Cheshire cat.
Haymitch from The Hunger Games would definitely be an interesting guest. He'd mostly likely show up drunk, carve his name in your grandma's mahogany table, and pass out in the desert. It wouldn't be dull, to say the least.
Scrooge wouldn't want to be at your Thanksgiving dinner, and he wouldn't be shy about telling you. He'd also complain constantly about the cost of the meal and how everybody should be working instead of eating. He'd be a real barrel of laughs. I'm pretty sure nobody would want to sit next to him.
Despite being awfully clever and a bit of a sociopath, he's also a bit of an ass. He might be fun to sit by--deducing what everyone's deal is--but you might find yourself wanting to stab him with your salad fork.
You might be wondering why you would even invite a cat to Thanksgiving dinner. But this is no ordinary feline. This is the cryptic and somewhat creepy Cheshire cat. His unusual aurora might weird you out a bit. Also, I'm pretty sure he'll eat your dessert when you're not looking.
Arthur Dent from Hitchhiker's Guide to the Galaxy would show up wearing a bathrobe and carrying a towel. You don't know if he's there to take a shower or eat dinner. Also he keeps bugging you about finding a good cup of tea.
Reepicheep from Prince Caspian would be an unwelcome addition. After all, who wants a rodent at the dinner table even if he can talk. Not to mention that he would challenge your honor and threaten to beat you black and blue with the flat of his sword with every attempt at conversation you made.
And finally . . . Dolores Umbridge shows up a half hour late. She's already mad because she has to dine with a bunch of muggles, but since she's late the only spot left for her is at the children's table.