^Just so you know, Warlock is not a term for male witch. It's a derogatory term for evil or bad witch that broke their sacred oaths. Which if Declan is the witch helping him, then he would be considered a warlock. As would Dahlia, Esther. It wasn't until turn of the century that due to Hollywood's use of the word that people started referring to all male witches as warlocks. Most find the term insulting. Witches is gender neutral.
Not to sure on the witch part, but I always pictured Declan turning out to be one of those supernatural hunters (the five) - if that's even still possible. And he activates it by killing Emmett. I'd prefer him to stay human, but if he had to be supernatural, id prefer hunter to witch.