I'm a developer and have messed with Homebrew games. With freemium being the future of gaining clearly who wants to remake destiny PvP as another fantasy scope based fps with supers double jump etc.. we the people can do what bungie can not. We can still discuss different game engines and different methods to do it since it's it's own game but thinking unreal engine would be the easiest approach for a free game that can be edited and updated w ease later.
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Not to be a party pooper but have you tried to ask Bungie if you can make a copy of their game. While you may use a different game engine and want to only do part of the game but Bungie owns all the proprietary and intellectual property covering Destiny and D2. Even if your game was free to play any content like Destiny means you are walking into a lot of legal trouble.