there really are a lot of choices
id either recomend using Either:
Tailoring/Enchanting
Allows u to make armor for yourself, plus stuff to fuel enchanting (By disenchanting green items)
Alchemy/Herbalism
my personal favorite, wont make money for you until much later or if you find the few choice items that sell. But this will GREATLY help with survivability, mana pots and healing pots are your friend, trolls blood will give u minor health regen and there are potions which will help increse DEF and STR a couple stats where while arent a main concern for warlocks, are always good to have.
Another you could try is Mining/Engineering
Gadgets are fun to play with : )
otherwise, go for something that will help fuel income on your main, IE if u make money selling weapons from blacksmithing, take up mining.
GL HF