The supporting cast for ENT was fine, especially if they had been given more to do.
The only two I'd replace would be Scott Bakula and Jolene Blalock.
Blalock lacked that intangible quality that previous Vulcans like Leonard Nimoy and Tim Russ possessed. Instead, she was a wooden board that really didn't improve until the final season. I don't know who I'd replace her with, but perhaps not trying to use her as a sex object and going older might have been the right move.
TPTB had the right ideas with Archer, but Bakula wasn't the man to pull it off. It was perfectly alright to me to have Archer make mistakes and not be as polished in space exploration or diplomacy as a Kirk or a Picard. This is where they needed an actor like Connor Trinneer minus the accent. A younger actor that could more effectively portray those characteristics.
I know I probably missed the "Just For Fun" part of this thread, but that is how I would recast ENT.
I didn't mean this to be a criticism of the ENT cast - therefore the 'just for fun' bit.
And for the record, I like Jolene Blalock in the role, minus the catsuit, and I don't think she was wooden at all, at least I thought she got a lot of Vulcan emotions across. Bakula... I agree that he didn't have the gravitas that the other captains did, but he did get better during the course of the series. I don't have a problem with him anymore as I did in season 1.
Archer - Keith Carradine
T'Pol - Jolene Blalock
Trip - Connor Trinneer
Hoshi - Grace Park (from BSG)
Malcolm - David Tennant
Travis - I wouldn't cast him at all... If we're gonna have a mute helmsman, lets at least get a busty chick to play her
Phlox - Billingsley was fine...
It'd be pointless to cast anyone as Travis unless the role is better written. I'd like to have seen the original concept for Travis - as an older, rough-around-the-edges Boomer who challenges Archer's perspective.