So do you want to make just a serious imperial stout, or a SERIOUS imperial stout?
Brown malt 25%
Amber malt 12%
Black malt 3%
I really like fawcett malts for the black, brown, and amber.
Shoot for OG 1.100, mash at 160°F. Yep, that high, you want this to finish chewy and sweet! WY 1098/1099 or white labs’ equivalent. I wouldn’t use S-04.
I’d keep IBU to the 60-70 range, with a healthy dose at 30 minutes. Something nice and classic like goldings. 90 minute boil helps get the high starting gravity.
You can complicate the grist with caramel malts and invert sugars, but why bother? The high percentage of toasted malts and high mash temperature will keep it thick and rich. Toss in some lactose if you want it more sweet, but this can be done at racking to secondary. Or toss in some oak, or bourbon, or vanilla. It’s a great base for all kinds of additions.