1. It would stand to reason that any Guardian could wield any element of the Light. Whether they can change at will or through some kind of ritual and rest period is unknown, but the demarcation seems strongly between classes, not subclasses, so there doesn't seem to be any reason that Guardians are locked into one from rebirth. 2. No. The abilities of each class/subclass are unique and help to differentiate and tell the story of each one. 3. While we don't know for sure, this is what I would guess (following the same order you presented): Defender, Gunslinger, Sunsinger, Striker, unknown what class he is, Defender, Gunslinger, Striker, Voidwalker, Bladedancer. 4. It is assumed that he died. It is clear that Toland was part of Eris' fireteam, and it is also stated by Eris that she is the only one that survived. 5. Rasputin's bunker and the design elements of PI also share some visual similarities. The legend of PI is that it came from before the collapse, and the flavor text says something to the effect of "you get the feeling this is less a weapon than a doorway". We know the Vex use transdimentional technology to power their weapons. It could be that PI was designed by Rasputin to use similar technology, which would explain it being the only rapid fire fusion rifle. It could also be that Bungie was lazy and just reused an art asset. 6. You are thinking too hard about it. This is just a cryptic description text on a cryptic weapon. 7. The Vex on Venus. Human scientists in the Ishtar Collective were studying the Vex before the Collapse. 8. Exo Guardian I would say are powered by Light, and I would assume Human and Awoken Guardians are the same (no need for food, water, etc. just sustained by Light). As for non Guardian Exos, I would venture some sort of sustainable fusion-based power source developed during the Golden Age. 9. One of the first Guardians. There are armor pieces for both Hunter and Titan bearing his/her name, so It is unknown what class he/she was. He/She is most known for cryptic dreams and prophecies, so perhaps he/she was not a Guardian in the same way we are. 10. It is a real thing. Ikora belongs to it. Since there is an armor piece called Heart of the Praxic Fire, I would venture that the order is made up of those who choose to use the Sunsinger subclass exclusively. 11. Theoretically yes. As I said before, it would stand to reason that any Guardian can wield and element. We don't see them of course, because Bungie has not created these third subclasses yet. 12. Osiris was exiled due to his disobeying the Vanguard and continuing his exploration of the Vex and the Darkness. Nothing is known about the Seven Seraphs, although I theorize that they are the followers/worshipers of the Warminds. 13. We don't know, but I would venture a guess that it is a manifestation of dark matter. Could be some element that the Traveler uses in it's terraforming process, converting the matter/energy of the vast nothingness of space (the void) into something useful. Hope this helps!

A little immature but still needing an answer is do exo's have penises and vaginas?

Shameless bump.

My thing is.... Do guardians get winded after a super use? Considering we have to build energy to use it again.