Should the King's successor be a son or daughter? 
Depends on what you are trying to portray.  See, I see male and female villains as different types of villains, with different things that make them despicable, but neither is more intimidating.  The thing I find scary about a female villain is how... base they can be.  See, men are typically men when they are villains (okay, that sounded wrong...).  But women take on both a male and female role, often, as villains.  They often act as a male villain, but are not afraid to use the powers of a woman to affect their prey, making them quite creepy, I think.  
Which is capable of being more evil? Male or female? 
Again, I don't think either is more capable.  Just depends on how you write the character.  I think typically male characters are more evil, only because people are more comfortable with writing wicked male characters.  But, as I said, I find a well done female villain gives me chills.
What about the villains in your book? What is their gender? 
Dunndar and Volgarin are male.  However, in my second book *will not say much so that she does not spoil things for her HW Beta Readers*, I introduce a complicated female character who is not really the villain, but supports the villain.  I find her callousness to evil quite disturbing.  
And, honestly, does King Dakia (pronounced Da-kI) even sound evil to you?  I would alter the spelling, and then yes, I would say it sounds good.  If you want to pronounce the name da-keye, I would write it Dakai.  The way you have it makes me pronounce it da-keya, which is not intimidating in sound, I think.  
Hope that helped. 
