As the title says what would the raven queen want in return for making someone a hexblade warlock 

I was think of something like being her hitman for awhile than keeping the powers as payment, but I would like to hear other ideas for this

You can play it any wqy the DM likes.

Perhaps just having you out there, being able to observe the world through your eyes (or your familiar's), is payment enough.

Perhaps she requires you to hunt and kill undead at least once a week in her name.

thank you for the ideas since I was just wanting to come up with some ideas to present to the DM and I couldn't think of any besides the example I gave

Perhaps she had a champion who was a cleric that while they are successful they aren't 100% effective. Perhaps she wanted to experiment and see what tapping into dark powers could lead too in a different kind of champion. Maybe you have disagreements with the cleric because often you are both orchestrated to the same quests by the Raven Queen. I imagine an antihero who uses darkness to fight darkness. I don't know I think a DM would have fun if you built in some sort of NPC that is your rival to provide plot hooks. 

I don´t think that´s how it works ... She is the Raven Queen and not a mid level mob boss. If You want her powers, that will cost you a lifetime so servitude. if you are lucky... 

I play a Hexblade Warlock with the Raven Queen as my patron and for me it is that she uses me as a weapon against any follower of Orcus.  Our DM has taken that part of my character and backstory and thus far through our first arc, the cult of Orcus has been a prominent villain.

According to MToF (chapter 2 on the Shadar-Kai), the Raven Queen likes to receive memories. You could be one of her agents sent to gather memories, maybe you got powers because she wants a specific memory, and charged you to get it for her.