I had this idea while drafting a different post but this deserves its own thread. It may not be something we could do in TERA but it's still a great idea (imo).
You start the game and create your main via the normal character selection screen. At this point you can only make one character (this is probably your main).
As the game story unfolds, around level 20 you meet your alt (it takes you to the character select screen where you make your alt). They join you and become your companion. You can swap between your main and your alt whenever you want.
Once you hit max level you pick up a third member for your account, which you create. They follow you around and perform basic functions. You can swap to them as well. While out questing you can get your alts to follow you and support you in combat (heal, tank or dps). They can also be tasked to harvest while your questing or craft. Once again, you can swap to them at any stage.
All your characters would appear as a portrait on the side of your screen, this might flash or be highlighted if they have a message from another player or they're being attacked. Click on the portrait and you are now playing that character.
In essence, you level all three. At max level, you might need to help your newest companion get gear or level up, which should be easy as you're max level - so you revisit the lower levels. New story chains unfold with each character, so what an NPC might say to your main might be entirely different to one of your alts. What do you think?