Skyrim needs a true hero, maybe such as himself
He's the hero Skyrim deserves, but not the one it needs right now.So we'll worship him. Because he deserves it.Because he's not our hero. He's a silent guardian, a watchful protector.A dead Nord.
Also a God caring enough to play an active role isn't something that hasn't happened before, hell Tiber Septim himself manifested himself as an Imperial named Wulf and gave you an item to help you kill Dagoth Ur in Morrowind. It isn't unlikely that he would intervene again to stop the Aludin crisis.
True. But really, sending a letter by courier? What is he, a God or the Dovahkiin's aging grandmother? As a God, if it were really him who sent the message, I'm sure he would do something much more badass. Think about all of the quests that involve divines/gods/etc in Skyrim. One talks to you inanimately through a haunted house. One makes you create some type of super-LSD substance in a pot and makes you snort it. Talos is not going to follow those acts up by sending a quaintly written parchment letter. He's going to zap you in the ass with a thunderbolt and tattoo his message on your rectum.
As for the Greybeards, there isn't any good reason as to why they aren't it except the whole telling you where two words of power are at the same time. Even if this was some "test", cause we all know how telling you where words are via letter instead of directly to you tests your abilities to the max, they continue the letters and directly telling you where the words are throughout the main quest and after.
I meant it was a test before (or for when) you even met them, you get the letter one quest before you're summoned by the Greybeards if I'm not mistaken. Also, I was not aware that there were multiple letters from "A Friend" besides the one.