There are games that have done cross platform before so it can be done. Biggest example in FF11, that players from XBox 360, PS2, and PC could all play together. They doing it again with FF14 for PC and PS3 / PS4. Now saying that Bungie has mentioned they have certain systems in place that are developed specially for each system. Cause of the way they did that the game will not be cross platform. Haven't heard it is cause of graphic unfairness. My guess it has to do with provided social software each system uses, Destiny takes advantage of it making playing with others easy. Not like you need huge groups for anything anyway, raids are only 6 man. Open worlds in beta were limited to 16 people and on Old Russia the most I saw in one area is 4. Yes even though you share your friend list on 360 and One, Microsoft uses different setups for matchmaking and friend list on the systems making them incompatible for game interaction.

In a Playstation Access video, DeeJ stated that cross-gen play was not possible, "like most console games." Later, several articles recounted what Bungie engineer Roger Wolfson said. The issue is not technical - Bungie is avoiding the potential backlash of gamers claiming that the current gen consoles have an advantage due to more power. I've included one of the articles. I understand the reasoning, but I think Bungie should test the waters before claiming they're invested with immature sharks. I particularly think that given that all versions will run at 30fps for parity. If the current gen systems are not going to be optimized for the sake of parity, I feel that play should be permitted within a console ecosystem. Without solid testing, we will never know if the difference of pixels truly matters. While not FPS games, several games have been successful using a model that allows play together on various platforms: FFXIV, DCUO, the upcoming Happy Wars is supposed to be the first between 360 & Xbox One, & Helldivers is supposed to support play between PS3, PS4, & Vita.