Well, it varies in different fanfics. The 11th Doctor could be hurt by powerful villains, or maybe in trying to save someone. It could also be from internal conflicts or emotional stress.
It could be due to various imaginative plot devices created by the fanfiction author. Maybe in a battle with a powerful villain or during a risky adventure.
The Daleks could be the main villain. They might be trying to manipulate the Doctor's regeneration to create chaos in the time - space continuum. Since the 9th Doctor's unexpected jump to the 11th Doctor disrupts the normal flow, the Daleks could take advantage of this instability to further their evil plans of universal domination.
It could be the Daleks. They are a classic Doctor Who villain. They might see the meeting of two Doctors as a threat to their plans for universal domination and try to stop it.
Sure. Although the 8th Doctor has a long and complex history in the novels, new fans can still jump in. For example, some novels start with a self - contained story that gives a good overview of who the Doctor is and what he does. They gradually introduce the lore and concepts associated with him, so new fans won't feel too lost.
The story could involve the OC having a special ability or background that complements or contrasts with the 10th Doctor. Maybe it brings a fresh perspective to familiar Doctor Who themes.
One popular character is Charley Pollard. She is a lively and intelligent companion of the 8th Doctor. Another is C'rizz, an alien with his own set of unique abilities and a complex personality.