I think that series 5 is going to be somewhat different from what the other series got us used to. First of all, it is supposed to take some years time skip, since Colin Morgan said that he won't have to shave daily in the new series in order to have Merlin look younger. That must mean that it won't necessarily be just a year after Morgana's second overthrowal, as usual. This also means that we will probably have a look at a more mature Arthur and wiser Merlin.
Also, Morgana will probably change tactics, since she couldn't prevent Guinevere Pendragon from becoming queen of Camelot. What I think is, she won't try to take the throne anymore, since it would be meaningless to be overthrowed for a third time. Probably she's just going to try and destroy the kingdom from now on. Aithusa and the villain returning will probably be a massive plot point as well.However she has to find a new home for herself, since her hut wouldn't be safe anymore. Merlin knows where it is after all. Maybe the isle of the blessed would be where she would head next.
In addition, I think that the funny Merlin and Arthur moments will be cut down since, you know, Gwen will probably look after the king and Merlin will be more of a friend and a councellor than a servant to him.
Although I think that most of us don't want that, Colin Morgan and Katie McGrath also said that Morgana has to pay for her actions, although they both justify them to an extent. This may mean that Morgana dies on the end or somewhere in series 5. However, noone can be sure about this, since the writers have said nothing like that. Also, an episode that Morgana seduces Merlin was discussed in an interview. I think something like that would be great to happen if she learnt the true identity of Emrys. Then, she could trap him in the crystal cave after seducing him, like how it happened to the legends, only different, because Merlin is not the legends.
What else new do you think that series 5 of Merlin will bring?