My Destiny 2 theory: The grimoire states that "there has always been a Speaker", the way that it says "A Speaker" indicates that it is not necessarily THE Speaker, or always the same person. This suggests that the role of the Speaker could be passed on from person to person, most likely by handing over the mask which allows you to communicate with the Traveller. Given the concept art that was released by one of the bungie artists of the city in flames it seems fairly safe to say that at some point in Destiny 2 the city will be attacked and probably destroyed. During this fight I speculate that the Speaker will die and in his dying moments he will pass the mask on to us, allowing us to communicate with the Traveller. This is where I think there will be a major twist; the Traveller says to you: "My friend. Kick back. Relax. Shrug off that armor, set down that blade. Roll your burdened shoulders and let down your guard. This is a place of life, a place of peace." In fact, the light and the dark are the same thing, all one entity. The Darkness holds the belief of the sword logic and wants to shape the universe through war in order for the strongest, most worthy race to rule. The Darkness created the light in order to drive conflict in the universe and create wars in order to eliminate the weak. The purpose of the Traveller was to give underdeveloped civilisations the help they needed to be able to compete, and then in order to test their worth the Traveller leaves them and beckons in the darkness to test them through combat. This would explain why the Traveller seems so ruthless for a being that is supposed to be a beacon of light and goodness, and would also tie in with the original Destiny story line that the Speaker was evil, because he was communing with the Deep.
Well the first part isnt speculation, it confirms that the title of speaker is passed down from one speaker to another. And that the speakers are chosen from the ranks of gaurdians. In one of the grimore cards it discusses how the previous speaker picked the current one, and of how osiris who was also competing for the title, left the city due to ideological differences with the current speaker. This is also partialy detailed as well as hinted at in ToO gear. Also partly because a now destroyed faction(GREEN FIST, the faction who's remains you see on the bannerfall map, i forgot their proper name) discovered and released documents suggesting that the ghost who chose osiris made a mistake and that he was an experimental golden age project, the contents of said experiment where unknown. [spoiler]i would like to note that some of the side effects of this were likely that unlike other gaurdians osiris was not driven by loot, nor the urge to follow orders, he was effectivley immune or resistant to the psychological tampering/profiling ghosts are known to do when they select people to be ressurected as gaurdians.[/spoiler]