Erandur comes to mind first:
Spoiler
The former follower of Vaermina turned priest of Mara. The quest he was involved in was great, he is voiced by the same guy as Joshua Graham, and he constantly comments about his life. He often says about how he has spent the majority of his life in Skyrim, but do we have the option to find out anything about his life during that time? No, we do not. At best he comments on how he was once a member of the Bard's College until they found out about his Vaermina worship. That's all. What about the challenges he faced growing up as a Dunmer in Skyrim? What about the details of his Vaermina worship and how he became one of his/her priests? What about his skill in magic? Why did he choose Mara over the other eight divines? Sure, I could fill in these details myself, but it just seems odd that Beth wouldn't take SOME time trying to flesh him out a bit more, he already has the skin, like most of Skyrim's character. But he also has bone, something that a lot of Skyrim's followers lack. But why not take the extra step and give him organs, a heart, a mind, a soul?