I think that we should be able to unlock Covenant abilities by completing story quest chains and gaining rep with the factions. So a system like this:
Regardless of what Covenant you join, you are still able to choose whatever abilities you want from any Covenant. That way choosing a Covenant is a purely visual choice, while gaining Covenant power is something that takes time to fully unlock but gives you a wide variety of choices in abilities when you have them all. Joining a Covenant gets you the the set of armor, but also an enhanced visual appearance for that Covenant’s abilities in a manner similar to Rank 4 Azerite Essences.
So lets say you have the four factions, Kyrian, Fae, Venthyr, and Necrolord. Just as an example, lets say each has an offensive ability, a defensive ability, a cleave ability, and a group buff ability. You start with one ability from each Covenant unlocked, but you get them in different orders. So you get the Kyrian group buff ability first, but their offensive ability last, while the opposite is true for Venthyr.
By ‘leveling up’ your Covenant rank with Kyrian and Venthyr, you gain an additional ability that you can use until you have all four. You are able to choose one offensive, defensive, cleave, and buff ability from any combination of Covenants. So if you want three Necrolord abilities but the defensive ability from Night Fae, you could do that assuming you have at least three Necrolord abilities unlocked.
This would essentially mean that we have a second ‘Talent Point’ system that is gradually unlocked over time by gaining rep and doing story quest chains. All players would eventually have all Covenant abilities unlocked if they keep getting rep and if they finish each faction’s quest chain.
And they could even fit the Maw in there somehow by making ‘Maw Challenges’ for each faction, like completing certain objectives within the Maw while only using a single faction’s Covenant abilities in order to upgrade them similar to Essences. So if you clear a run of the Maw while only using Venthyr Covenant abilities, you get slightly upgraded Venthyr Covenant abilities.