Apart from Kalonzo, Martha Karua is also a potentially good running mate for Raila because she would attract most of the older Kikuyu voters who have stuck with Uhuru. She would assuage their fears about Raila. However, I highly doubt Uhuru would allow her to be the running mate.
I'm beginning to think that Uhuru doesn't want Raila to win. This is because a Raila victory will render Uhuru as Azimio Chairman weak since he won't be having State Power to protect his loot. He will be among the first to be cast aside.
However, if Raila loses, Uhuru will be powerful as Azimio Chairman. Raila won't be having state power to get back at him, so he'll be able to control Azimio and its MPs so that President Ruto cannot easily go after him.
He will pretend to be campaigning for Raila, keep promising to go to the "ground" in Mt Kenya but never actually doing so. He won't want a strong DP for Raila, so he might go for a nonsensical choice like Gideon or PK.