Well, we've had one on Venus, one on the Moon, and one on the Dreadnaught. So that leaves the Earth, Mars, and I guess maybe Mercury as the locations we've been to before that don't have raids. I think Mercury is unlikely, as that's a Vex world, and we've already had a Vex raid. Mars is possible for some kind of Cabal raid, but I think a Cabal raid would be better if it was set on some kind of huge warship. Perhaps the Emperor arrives in our solar system on his flagship, and we have to take him down (although that's kind of similar to Oryx and the Dreadnaught). I think one of Earth would be neat. Rasputin would be cool, but we've done a fair bit with him already. Personally, I think it would be awesome if the raid took place somewhere on Earth we haven't been before. Like maybe Antarctica, or the Sahara, or something; an area with a distinct visual style. Maybe the Fallen have found some sort of super golden age tech hidden away in this remote area, and we have to go get it back from them.