The trouble of having characters we already know, is that we know their destiny. For me that means there will never be any real jeopardy. You know young Aragorn will triumph and survive even if faced with the Hordes of Moria alone. Unless they kill of Aragorn in the first show and change Middle Earth's future forever, the entire show could feel like an extended flashback for the LOTR books and/or films.
I'm more concerned that they'll "pull a Gotham" or Smallville What I mean is, if you have a story featuring Aragorn and Sarumon and Gandalf and Sauron, the pressure will be on to push them to their destiny as the characters we know later, regardless of how much it makes sense with the established lore or the timeline. The characters have to develop, and if they don't give their characters enough room to develop, they might develop a little too much.