Summary of events: The Marksman killed Sundance, RiP. Uldren killed Cayde, RiP.
Facts; Sundance is gone. Cayde is dead. Cayde was never drained of his Light, like the victims of the original Thorn, like Jaren Ward. As long as a Guardian is not specifically drained of their Light, they can be brought back by a Ghost. Any Ghost.
Lore Facts;[u] Shin Malphur was killed as a child, and almost immediately resurrected by a Ghost.[/u] This Ghost then sacrificed it self by leading Fallen away from Shin's group. Shin goes on to grow up in a town called Palamon, where he meets both Jaren Ward and Dredgen Yor. After Yor destroyed Palamon, a small group of survivors, including Shin and Jaren, track him over a period of years. One night Jaren sneaks off to have a stand-off with Yor alone. [u]Yor drains Jaren of his light, but doesn't kill his Ghost.[/u] Instead, he tells the Ghost to go to "The Other" and take him "Jaren's piece," The Last Word. The Ghost immediately knows Yor is referring to Shin. The Ghost finds Shin, and [u]adopts him as her new guardian.[/u]
What Bungie have established through this lore is that Ghosts are not irrevocably tied to a single guardian. If a Ghost's guardian is made mortal and then dies, then the Ghost can take up another guardian, possibly finding a new one "from scratch." So all we need to bring Cayde back is to find a Ghost who's guardian was made mortal, and then killed. I'm sure there are many after the Red War, but I can think of a more poetic way to bring him back. With Dredgen Yor's Ghost. Or to be more accurate, Rezyl Azzir's Ghost, the man who was corrupted by a Hive Wizard and turned to the Darkness. Yor sent his Ghost away, ordering it to go and tell of the man he had become. In theory, Yor's Ghost could resurrect Cayde. But even if he couldn't, ANY Ghost who's guardian was made mortal and died could adopt Cayde.
TL;DR: Cayde could be adopted and brought back by ANY Ghost who's guardian was made mortal and died, just like Shin Malphur was adopted by Jaren Ward's Ghost when Dredgen Yor used Thorn to drain Ward of his Light.
Alright, this might seem too simple, but aren't Exo just sentient machines? There must be some sort of memory in every exo, meaning they could be downloaded and installed into a new body. There is a chance that Cayde could possibly be rebuilt. Now the argument against this is the fact that exos were built thousands of years ago, so it would be hard to build one. But there are pretty many exo guardians which can be sccanned and reverse-engineered from scratch. There could also be sealed off ancient exo factories, which could contain blueprints and perhaps even a finished body inside.