I think Jon Hamm would be an excellent Solo but, leaving aside the unlikeliness of two leading men called Hamm and Hammer(!) toplining a movie, I think the studio will be looking for a big-name star to add weight to the as-yet-unproven Armie Hammer, who plays Kuryakin. The Lone Ranger is likely to be a hit but it's Johnny Depp who will be putting bums on seats, not Armie and I think that this film will follow a similar path of pairing him with a proven draw.
George Clooney, previously in the running, is probably too old to co-star with Hammer - they'd look like father and son or uncle and nephew.
With Guy Ritchie having given them a hit in the Sherlock Holmes movies, might they go back to RDJ (who wouldn't be perfect for the role but would be as good as Cruise, IMHO) or has he already too many franchises to juggle?