How to create a hexblade: theoretical class/theme and background combinations

I really like what you can do with the combination of classes and themes and backgrounds. From the playtest, you can see what kind of radical different types of characters you can make with the cleric class by taking different themes. I was wondering, how easy would it be to make a class like the hexblade? Basically a martial warlock/wizard type character with these different mechanics. 

Would you take the fighter class and add a different theme, such as "magic-user" granting you the use of some minor spells? Or would you go the other route and try to make a class such as the wizard (or warlock/sorceror) more martial.

I'm rather excited about the amount of creativity this system allows, as you no longer need a class for every concept.