They’ve done peaches. Chen Kenichi won.
Varmint.
Popcorn and velveeta were my first choices but I think Twinkies would be fun.
Or, sugar. Just sugar.
Tasso
Scotch Bonnet peppers
American cheese
Captain Crunch.
Wax fruit
pork rinds
Pixie Stix
A still living vicious dog
One of the other chefs or The loser(s) of the previous episode.
They’ve done natto, too - perhaps the finest, most theatrical introduction of an ingredient Kaga ever did.
Popsicles.
oooh, I love the circus peanuts and pixie sticks.
I’d go with pickled pig’s feet or Cheese Whiz.
Pocky
Suet
What about The Chect, or Strong Mad 
Tabasco Sauce
Wild Turkey (the liquor, not the animal)
Flour
Sweet Tea
Doritos
Something from the bottom of the Marianas Trench. But it must be cooked within 20 minutes 
Marmite 
Fruit Roll-Ups
Peanut Butter Cap’n Crunch
Salt water taffy
Leftovers from Fear Factor
100% genuine imitation saskatchewan sealskin
wascally wabbit
dehydrated water
toenail clippings (fresh)
I imagine walnuts would be pretty difficult.
Pigs feet, actually would be worse.
Pig – long pig, that is.
Or if that’s not suitable for cable TV, then how about Anomalocaris.
Hare Krishna
cream cheese
Lavender
Lemons
Yum!
Grits…definitely grits.
That secret ingredient would definitely lead to some seppuku right there in Kitchen Stadium.
[Monty Python] OK, this potentially terrific thread has become silly. All silliness must immediately cease and desist. [/Monty Python]
If the OP has no objection, may I suggest that nominees be limited to plausible ingredients in a main course?
My nominee would be road-kill squirrel - people actually do cook with this…