That's a tough one. I once heard an idea that the characters of Jason and Cecil be combined, and having them be "next door neighbors" to the Carringtons, so to speak. I'd be tempted to get rid of Bliss' character altogether, if it weren't for the fact that she too had her potential. Plus it would be a different actress playing her as well.
But as for casting...
For a while now, I've become fond of the notion of Marcia Cross playing Sable, with Kristin Davis as Francesca. Both women were on the original MELROSE PLACE, and each ended up on different-yet-similar shows: DESPERATE HOUSEWIVES and SEX AND THE CITY, each of which featured a group of four women. Both series were successful, and it would be interesting seeing these two working together again.
Not only that, but one of my top choices for Cecil is Kyle MacLachlan, who was not only on both DESPERATE HOUSEWIVES and SEX IN THE CITY, but was also a love interest to both Cross' and Davis' characters. If they didn't combine Jason and Cecil's characters, it would be kind've funny seeing both women now battling over Orson's/Trey's brother. lol
Also, whenever I think of possible candidates for Miles, I always think of Trevor Donovan, who starred in the new 90210. Although he's in his 30s, so he could be way too old for the part, though he might still be able to pass for mid 20s, since I believe that was supposed to be Miles' age anyway.
