Tom Cruise as the next Napoleon Solo doesn't quite compute.
Who COULD do justice to that role among today's actors?
Robert Vaughn brought a lighter touch and enough of a sense of humor to the role to make his character believable even in an unbelievable situation, without taking himself too seriously--as did David McCallum doing Illya Kuryakin. You need someone with the same skill sets they had, to bring the story off now. Otherwise a light hearted spy spoof like the original series was--and this film should be--can be weighted down into a dull adventure yarn.
These days McCallum has other things to do (every week on CBS) and Vaughn's also busy with stage work and recurring roles on the BBC, although perhaps Vaughn could play Alexander Waverly if he has the time to fit it into his schedule--he is about the same age Leo G. Carroll was in 1964.
Too bad Clooney doesn't feel up to doing Napoleon Solo, he'd have been a perfect fit. Maybe Brad Pitt could handle it. Can't think of anyone else off the top of my head.
If Pitt's not available or interested, they'd be better off scanning today's sitcoms and comedy/variety shows, or the Broadway and British stage scenes (since the stage is where both Vaughn and McCallum got their starts, and where both men learned how to play a variety of characters, situations, moods and genres) to find their next lead actors.