Megurezza wrote: »

I know that when you finish the main quests, you can go into another faction's territory.
BUT
I love doing achievements, and I saw that some of them requests to complete something in ALL factions (for example explorations' ones)
So, are they impossible to complete right now?
Will they add the possibility to go there in the future?

I dont understand what you are saying. You go to all zones during cadwels silver and gold.

Megurezza wrote: »

I'm not a veteran yet, so I asked to other people, and they told me that you can access just 2 zone, your starte and the one you choose after
So they told me wrong?

Yes, you were misinformed. When you complete your own alliance's zones and the main questline, you then move on to your second alliance. This is referred to as your Cadwell's Silver alliance/zones. You don't get to pick which one you go to second - you basically go around Tamriel in a counter-clockwise direction. So it goes DC > AD > EP.

After you complete your Cadwell's Silver zones, you move on to the 3rd alliance. This is your Cadwell's Gold alliance/zones.

So you do go through all of them, and you don't get to select the order (aside from your initial choice of home alliance).