I'm not positive any would have benefitted from extra time. Even the most plot-heavy episodes seemed to spend enough time showing off the characters and savoring the mood and, where they were good, exploring the ideas developed that the production didn't feel cheated, and even the most mood- or character-heavy episodes kept the plot moving at a fair pace.
The Original Series did extremely well at balancing all the aspects needed to make an interesting show, and while we might see situations where we'd like to know more about the goings-on (e.g., Landru, Eminiar, Yonada or the workings of Vulcan's mystic/ritual side) I'm skeptical that there could be that much more told about them without the explanations becoming too nonsensical or the episodes becoming leaden.