Hello @ZooSam, I think this is an awesome project! I would absolutely love to see a Cayde movie—or anything that involves Cayde if I’m honest—and I think it’s such a cool idea.
Being a six year Destiny fan myself, and a long time writer as well, I have written a lot of fan fiction on the subject of Destiny and it’s characters. And I can tell you with confidence that it is a ton of fun!
And you asked for some ideas on how to start, so here’s an idea:
I think a good way to start—and quite possibly the only way to start—is with Cayde’s resurrection at the hands of Sundance—And yes, I know Sundance doesn’t have hands with which to do anything, but just go with me on this one, alright?
I’m positive there is a lore card that describes Cayde’s first moments of life in detail, so I won’t describe it all here. However, I’m blanking on the name of the card right now, but if you look it up I’m sure you can find it.
Anyway, He’s all resurrected and stuff, and he maybe finds a note in his pocket—one that he left himself long ago in a life long past. As Cayde is very well known for having left himself notes in order to help his future self be better than the previous one.
And maybe the note could have coordinates written on it, leading him to some secret chests that he hid in his previous lives—and those chests will have more notes describing who he was, the things he did, and who he should be now. And maybe they could tell him about his Ace and his Queen, and why they are important to him—btw, if you haven’t read “The Man They Called Cayde” lore book, then read it. It’s so good!
So basically the first part could be about his journey to find the notes left by Cayde-5, the adventures of a life in the wild frontier, and a tale of how he grows accustomed to his new life as a guardian. And after he sort of learns who he is, he could venture out to the city—which would not have even been built yet—And that would set him on a path to meet many people, including Zavala, Ikora, Andal Brask and the rest of his crew. And that would set Cayde and his on a path of loot, plunder and exciting tales of ferocious battles. Until of course Andal dies by the hands of Taniks the Scared, and Cayde takes up his cloak, and the mantle of Hunter Vanguard.
I won’t detail all of my thoughts, or this message might turn into a book all in itself. But hopefully those ideas can get you started on something really cool 
thanks for letting me spill my creative juices everywhere lol.
Also, a screenplay about The Drifter sounds just as cool! There just isn’t a whole lot that we know about his past. So it might be kinda hard to write about him. But I’m sure if you dig deep enough, you can find quite a bit of lore on him.