As well as Artifacts for each class specialization, there will also be the addition of the Class Orders. In Legion, you won't be fighting the invasion of the Burning Legion alongside the Alliance or the Horde, you will be fighting them with the power of your class.
With the armies of both the Alliance and the Horde shattered after the Iron Horde and the events of Pandaria, the armies of Azeroth are in no shape for yet another war. As a result, we will band together with our fellow class members to tackle the threat of the Burning Legion in ways unique to our class and to the advantage of our strengths.
Regardless of faction, Class Order Halls will be places to be shared with fellow Class Members; no more personal Garrisons. For an idea of how this might look like, look to Acherus as the closest thing to a Class Order Hall.
It is in your Class Order Halls that you will be able to customize your Artifact weapon and get quests. Order Halls will not have banks or auction houses. The order hall for Paladins was shown at the Gamescom reveal, and Warlocks will have their Order Hall on a Legion portal world. As for the other classes, we can only speculate that Acherus will the order hall for Death Knights and suitable areas for other classes.
You are able to collect members of your class to fight the Burning Legion, as well as Elite lore figures such as Darion Mograine. The Elite characters are far stronger than simple followers, and you are also able to send these characters to patrol zones. Champions are intended to enhance your game-play experience, rather than going out and doing things for you. Examples include assigning a champion to a zone, and whilst you are playing in that zone you will either gain combat bonuses or rewards.
It should be noted that Champions are different from followers, so they do less of an impact, and they enhance things instead. You can send them on missions, but they are meant for much more. Blizzard would like to avoid giving out too many followers, because they gave less reason to go out into the world and accomplish things. The size of your elite squadron, will be about nine, think Knights of the Round Table.
Although the fantasy for Warlords of Draenor was that you raised an army and felt like a leader, the fantasy for Legion is that you work alongside elite heroes against the threat of the Legion. As such, amassing huge numbers is not a solution.