In terms of playing "the original way" (the original way is NOT REQUIRED) for you or any new player who is curious and who has purchased and made to start in the zone of a chapter like Morrowind or Summerset, you would take an NPC boat or get a friendly player to take you to a zone like Auridon, Glenumbra, or Stonefalls (other base game zone may work as well? not sure?). A mysterious Hooded Figure will approach you on the docks. Follow that person and it will start the game's main questline. That will take you to a new zone for a brief adventure that used to be a tutorial, then you will be dropped onto the starter island for your faction (Khenarthi's Roost for AD, Stros M'kai for DC, or Bleakrock Isle for EP). You will there pick up a faction questline that you can follow from the starter island(s) to the five main zones of your faction while simultaneously doing the main storyline for the game. Eventually both of those storylines will end in far-off place, leaving you with lots of options for what to do next.

Again, the above paragraph isn't how you have to play, it's just how the base-game and its storylines work, so that's the order that makes the most sense if you are trying to follow along. It will also give you chance after you complete the main storyline and your own faction's storyline to have a lore-based excuse for doing the storylines of the other two factions. But again, with One Tamriel you can just go to another faction's zones anyway whenever you like so it isn't necessary to follow the original pathway.

(FYI Craglorn was a zone you originally did after completing your Alliance zones that was partly released early on and finished a few months after game launch -- it was the last part of the original game release. Similar to the post-base game zones that would follow with the introduction of DLC, it is self-contained in its story and questing.)

The release of new content after base game so far has been:

• The Imperial City - new zone (PvE + PvP)
  
• Orsinium - new large zone
  
• Thieve's Guild - new zone
  
• Dark Brotherhood - new zone
  
• Shadows of the Hist - new dungeons
  
• Morrowind - new large zone
  
• Horns of the Reach - new dungeons
  
• Clockwork City - new zone
  
• Dragon Bones - new dungeons
  
• Summerset - new large zone
  
• Wolfhunter - new dungeons
  
• Murkmire - new zone (just released on October 22nd, 2018 for PC)

Again, these all have self-contained stories so you can do them in any order or not at all. Some NPCs from the base-game will have a line or two of dialogue "recognizing" you if you met them in the base-game quests but that's it. Technically there is a story arc that spans the very last dialogue of Orsnium, gets going in Morrowind, heats up in Clockwork City, and ends in Summerset. So that sequence makes the most sense for those zones. BUT, they made it so you can still play those zones in any order.

There is also PvE and PvP in the Cyrodiil zone (released with the original base-game), which you can access starting at level 10. It's basically a PvP map for siege warfare, open world battles, etc., but it is stocked with PvE quests and objectives as well. You can also try Battlegrounds (added later to the base-game) at level 10 for a faster, briefer, smaller PvP experience.