Well, there are some pretty amazing mods out there which have entirely new dialogue and quests involved, but whenever I play them I feel horribly un-immersed due to the lack of voice acting alongside the rest of Oblivion's massive amount of it.
But it's very unrealistic to create entirely new voice acting for every mod. So I suggest either a modders' resource or automatic silence-override mod. It would contain lots of voiced/lip-synced bits of speech and "grunts" to fill the void.
Lots of, "Uh..." "Well..." "Hehehe!" "Argh!" "Ugh." and "Huh?" things. Perhaps also based on disposition.
They could be derived from Oblivion's voice-segments, and cut in certain spots.
While it wouldn't be the best voice-work, it would certainly fill the painful silence that many mods suffer from.