I'm fairly certain it will be the exact same system as in ShB. You can use them when they are relevant without issue. First everything you need to level them. I prefer that system, honestly. We don't need more incentives to not MMO.
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I'd assume it's going to be just like Matoya's Relict and Paglth'an.
Scenario keeping with lore where only those available in the story can go, and Avatar as non-canon can do whatever.