Depends on the story-line being continued/extended/added. Some areas really are suited to being continued, and I really like that. But don't continue something that doesn't make sense.
Adding quests ontop of vanilla content is also fine - e.g. A quest to kill Maven would be appreciated

As for the voice acting, generally you either do the quest without voice acting, or get someone who can imitate vanilla well. Which is hard to find, thus more "Moonpath to Elyswer" style quests than "Kill Maven Blackbriar" quests.