So, the dawnguard were formed in the late 2nd era after the jarl of the rift had his son go down into the vampire pit currently in the ratway, become infected and then become jailed.
As much as there is still 300 ish years left of the 2nd era it would be cool to see the very beginnings of the faction, as well as potentially early prototypes of crossbows that aren't a single ability.