I don't know. I've been seeing level 8 dudes loitering around the Orsinium and Thieves Guild map. You are pretty much will be buffed to match the Zone's level.

But I think it's better to stick to your main alliance zones, and once you finish that you can start hitting the DLCs. Why? Because you wouldn't want to be overgeared while doing your main alliance quests, since it's not scaled and you will one shot everything and got crappy level 10 gears just to throw away.

But I guess it's all wont matter once Tamriel One hits, you can just do whatever you want.

I would suggest you don't try to do Thieves Guild and Dark Brotherhood all at once though. The jobs you need to do to earn Reputation can get very repetitive if you're not doing anything else. Leave Orsinium till later, but you may want to start dipping into Thieves Guild and Dark Brotherhood as you go along.

Overlevelling is a bit of a problem now, but after October that will no longer be an issue.

October update will cause all zones to level with you not just the DLC content. With your first character if you are interested in the stories or the lore I would suggest doing thing in order. Do the alliance quest line then the silver then the gold and along the way throw in the main story and guild quest lines. Some of the later content you will run into NPCs you met earlier in the game and if you like the story that is kind of cool.

If you don't care about the story line then jump all over the place. If some quest line is becoming a grind just back burner it for a while and do something else.

If you wanna follow the story, you should probably wait and do all but the Imperial City DLC last.