No need for spoiler tags. Probably for aesthetic pleasing

OFF COURSE New Rockstars made a video about this 7th seat and it's probably for either Namor the Submariner or T'Challa of Earht-838. It can't be for 838-Strange, because 838 Mordo took his place. Superior Iron Man also not, because Captain Carter took his place in the Illuminati as the representation of the Avengers.

We've got Professor X, Captain Marvel, Mister Fantastic, Baron Mordo, one of the Stranges and Black Bolt. The last chair is reserved for Pizza Poppa.

(joke)

^ not a joke. Very serious.

To my knowledge, a rejected version of the film would have Balder sitting in the chair.