Simon MacCorkindale certainly was around for a short time; I, too, thought he'd be around longer but was gone in a flash.
I always tend to think that France Benard's Count du Chalon (from the La Mirage grand opening) who knew Kirby from Paris, could have stuck around longer -- perhaps replace or supplant PeeWee Devilbus (yes, I agree repugnant Helmut Berger was perfect as a slice of opportunistic EuroTrash -- but only if the show were better produced; as it was, Berger's casting just comes off as yet another bungle). And later, Count Pierre would have some lineage association with the Moldavian royals.
Grant Goodeve's gay lawyer, Chris Deegan, should have been around longer, too. I have, in my head, a Season 8 story arc in which Chris is diagnosed with H.I.V., and Steven immediately becomes concerned about his own health because, as it turns out, Chris was the late Ted Dinard's lover before Ted started dating Steven in the late-'70s... The family (probably because of evil Adam) finds out, and Steven is reluctant to take a blood test; Blake, fearing he may soon lose his youngest son, warms up to Steven; Alexis, curiously, shuts down to him (temporarily), afraid of the loss. Ultimately, Deegan winds up in a hospice, deteriorates and passes away before the end of the season, and Steven is revealed to be H.I.V. negative, and then he (and whoever his lover is by that point -- I really don't want Bart to be it) leaves for Washington, DC to be a lobbyist).
The Duchess of Brown Out would have been a player in Season 6, and then I might spin her off with THE COLBYS (which would happen later in Season 6 than it did). Two or three Moldavians would follow the spin-off, perhaps including King Galen (whom I would blend with Zach Powers' character, with Montalban cast from the very beginning).
Believe it or not, I would have kept slimy Nicole Simpson around for the remainder of Season 5 (Jeff, much like Bobby Ewing, always needs a lot of stalkers) who follows hm to Europe and is killed in the massacre.
I'm not sure if this is completely on-topic, but I'd combine Lady Ashley's character with Frankie Scott's character -- and perhaps make her Blake's former fiancé whom a teenaged Alexis once shoved out of the way back in the '50s (although Alexis claims nonsensically it was in the '70s). And if Lady Frankie perchance survives the massacre, she could use her Georgetown connections to act as a political mentor for Blake during the gubernatorial race in Season 8.
Obviously, after Caress Morell disappears (along with most of the Fallmonts) during the avalanche cliffhanger which closes Season 7, she would pop back up in phantom Season 10, and would be revealed to be the shadowy figure who'd pulled Sable into her Season 9 search for The Collection after Jason finally dumped her (based on an envelope-of-stuff Agatha Scott, the mother of Sable and Frankie, gave Caress during Season 6).
Then, of course, The Monster Bitch of Season 10, Mother Blaisdel, intent on destroying the Carringtons once and for all, whom she blames for the semi-deaths of her son and granddaughter. (Mother Blaisdel is probably best buds with Eva Braun anyway).