As to the Tonight Show, whenever Jay Leno finally retires (my guess is when he hits 65 in about three years)......
As I said in another thread a few weeks back, I highly doubt Jay Leno will retire anytime soon. He's too addicted to the 11:30 joke-telling limelight.
Whenever I think of Arsenio, I cannot help but think of the early '90s SNL skit with Dana Carvey -- "Carsenio" -- and Phil Hartman as Ed McMahon with a fade haircut. "Stay tuned for Ed McMahon's PARTY MACHINE!"
Personally, I think Arsenio Hall - mellowed with age - could pull it off. We'll see.