So I got to thinking what don't the Illuminati cover? I see them as both pre-empting threats - cos threats always come from somewhere - and also having a pool of near infinite resources. Like this:

Cap - every super hero listens to this guy. He has unparalleled access both to the Avengers data and Shield's data.

Ironman - Technology.

Xavier - Unconnectedly, the mind and genetics.

Strange - magic / mystic arts.

Richard - the real polymath, covering 'Science' but also, time travel, all the non-human races that share our world, space, other dimensions, maybe alternate dimensions.

OK - there is some cross-over there. My question is: if they added someone, who should it be and what would they have to bring to the table?

No one on this team speaks for Marvel's pantheon of gods. That, and the other untapped source of problems / resource is Marvel's realm of the dead, leads me to: Valkyrie. She would also provide a much needed female perspective to the team, the female psyche being an unfathomable threat and untapped resource for the current roster.

They were all originally scheemers and manipulators.

Iron Man
Mr Fantastic
Dr Strange
Namor
Black Bolt

And invited but declining - Black Panther, all have one single thing in common: Schemes and plans, a grey sometimes elastic moral.

Cap is the first guy in that group that isn't a natural scheemer and manipulator, the odd man out, so to speak.

As to what the group is missing, Black Panther actually.

International pull and experience, to influence governments, trough manipulation and scare-tactics. He knows that political game real well.

I suppose some would say Black Bolt covers that, but the truth is that he's tried his best to not play that game. That's why he moved his city to the moon.

If you're going to put someone on the team, then it has to be someone manipulative. There's a truth to the godly aspect I suppose, so...

Kid Loki?