There’s a surprising twist at the end of this series that becomes a big
part of series five. It’s fair to say that a character we’ve met before
returns, an evil character, and I think people can probably guess who
that is.

Nimueh, Morgause or Mordred?

The returning evil character will likely be Mordred but I’d have liked it to be either Freya, Uther, or Nimueh.  Using the Hero Archetype as a basis:

Merlin turning bad?  Yes!  But, probably not.

A lot of Series 4 pointed to that… the growing isolation, the frustration, the emotional repression, even the growing social inequity (not only between Arthur but with Gwen and the KORT).  We’ve seen bits of Merlin’s resentment (his attitude toward Gwaine in the Sharer of Secrets and to Gaius in the Servant of Two Masters).  We’ve seen what some of the knights probably really think of Merlin (Sir Leon’s statement in Lamia).  And, his outlets to vent his emotional conflicts have dwindled.

If the writers and Colin Morgan want to have a severely emotionally damaged Merlin, the time’s ripe because his mental health is slipping by whether he knows it or not.  This will only grow and worsen over time to the point that something bad will happen.  Merlin would become what he despised and fought all these years— a sorcerer using his powers for selfish reasons like Gili, Nimueh or Sigan.  It would be interesting to see how Merlin wold cope with that but he’d likely be embittered.

But, since we expect that then Merlin embracing the Dark Side during S5 isn’t likely.

Morgana turning good?  Yes.  But, probably not in S5.

Morgana has committed nearly every evil act in the Merlin-o-verse.  13 episodes for redemption just would not be enough. Besides, you’d still have the Merlin problems.

Mordred returning?  Too, bad.  That’s likely the case because I can see a story arc where Arthur adopts the Druid boy as a show of Tolerance toward the Old Religion.  Then, Mordred replaces Merlin as Arthur’s best friend/advisor while plotting to overthrow Arthur, kill Gwen and rule Camelot (sort of like The Omen?) but being thwarted by Merlin at every turn until the episode where Arthur finds out Merlin has had Magic all these years and banishes Merlin from Camelot forever.    

Using the Hero Archetype there are other exciting and innovative ideas to utilize that haven’t been explored yet so I doubt it would happen now because it involves the use of prophylactic, abortive, and redemptive arcs.