I am n00b in this forum, so my ideas could be at somewhat misinformed.
I saw the wallpapers and a couple of the forum post, and I this is my personal speculation but I read somewhere that each of the three main characters is going to have a challenge. If I have to guess each challenge should correspond to the character itself. Hence, Rosella being the most “immature” out of all the character is probably going to make the “mature” or hardest decision because it is what correlates to each character.
I see Alexander as a romantic figure, in terms that he is the long lost prince and the prince that rescues and damsel in distress. Romantic people are portrayed in a good light however, not everything about them is good. Romantic people tend to be very emotional and sometimes loose reason, Alexander being now in a position of authority cannot let his emotion get the best of him or hence he would lead his Kingdom in to disaster. Cassima might be a part of this, because someone light and inspiration does not mean that is a positive light. Remember Alexander fate is to redeem Cassima from darkness it does not mention how he accomplish, for all we know Cassima might still be surrounded in darkness or Alexander traded his place for Cassima.
The whole Master of Darkness thing does not necessary mean that he is evil, remember Alexander had a dark past and he might have some dark tendencies, conquering this darkness might let him to master the darkness.
If I was writing Alexander’s challenge for the next games I would have to believe that it might be more a internal challenge with his past and possibly Cassima darkness. The question that I would answer depending how each person play is he strong enough to win against that darkness.
In summary, I would have to say that Cassima role might be paramount to make Alexander face his darkness either by his past, his feelings or both.
Either way I can’t wait for the release of the game to see the details.