The problem with copy already made game art is what you're modeling has already been made into a model and all you're doing really is mimicking their same moves. It'd definitely help to add your own spin on to it, and like you mentioned use outside references to better bungie's models. But I still feel as though it'd be more beneficial to make something based off a reference that isn't an in game model. If you like bungie's stuff, perhaps you might try and find some of their concept art from like the original Halo days and try and model something that never actually made it past the concept stages. This way you get some experience with using your own creativity to better it and you're working from something that isn't already a game model. Really either way works, this is just my opinion on how I think you can better your skill. Which 3D modeling is more of a tool set skill than an art, for most anyway.