When the series was just starting, I was either here or at another site, and did a tongue in cheek proposal of William Shatner as an angry Klingon determined to destroy the Enterprise. With Jeffrey Combs adding yet another character to his long list as his lieutenant, who was named K'Ching. I don't think Combs had appeared as Shran yet. In the same post, I suggested Combs in an episode playing 7 different roles.
I don't think anyone would've bought Combs as a Klingon
but that's okay because he created a character who was tough in his own right.
I would've to have a couple of other people who'd guest starred on the original series in new roles. There aren't that many who were still working into the 21st century. Besides the three that did
appear on "Enterprise", the ones I could think of were Lee Merriweather (Losira), Michael Forest (Apollo), and Phil Morris (the latter, I assume would've been the most likely to come back).
Besides that, some other people it might've been nice to see again were Judson Scott, Carolyn Seymour, W. Morgan Sheppard, David Warner, Harris Yulin, Lawrence Pressman, John Glover, Alan Oppenheimer, Robin Curtis, and James Sloyan.