Back in Shadowlands I tried to think about a way of making things make sense with the Titans and the Shadowlands. my idea was this:
Imagine that when the Titans started to order the cosmos they needed to build an army to keep the planets they visited ordered or to nurture the worldsouls until they become titans. In order to create their titanforged (keepers, etc.) they needed a specific magic to ''animate'' those armies, Anima (this way they can link the Shadowlands anima with Ra-den's anima from Throne of Thunder). The problem is that anima got sucked automatically into a pocket dimension (Shadowlands) making it impossible to use outside of it, so they followed the trail and found the Shadowlands.
The Shadowlands were pure anarchy, complete chaos where souls devoured other souls, full freedom but no ''safety'', the covenants where created naturally through the union of different groups to protect themselves from the chaos, banding together based on specific traits, nature-aligned beings created Ardenweald or honorable warriors created Maldraxxus.
When the titans arrived they basically stomped everyone and told them: Look, wee need your anima to do our thing, so we are going to create a gigantic, spell-scroll of stone (Oribos) at the center of your realm to stop it from sucking anima from the living world.
The titans need to keep Oribos safe, so they make a deal with the covenants, the titans will offer an ordered system without chaos and destruction, a system where everyone gets anima through a machine they leave (The arbiter), and each covenant will get a purpose.
- Kyrian - They will gain the ability of traveling between the Shadowlands and the living world to ferry the souls of the dead, because now that the Shadowlands don't suck anima automatically, the souls of living beings stay in the living world. This would be why they need to get rid of their memories and past life, so that they don't get tempted to stay in the world of the living.
- Ardenweald - These guys would regenerate nature creatures, sending them back to their worlds, as some kind of fail-safe measure, because ancients/loa are supposed to nurture worldsouls with spirit magic (something like that is written in the Chronicles vol. 1), so if these powerful nature creatures stay alive and go back to their worlds maybe one of those worlds has a worldsoul and they can keep nurturing it while the titans look for it around the universe. This could explain the connection between the Winter Queen and Elune, it could have been done through Eonar.
- Maldraxxus - The shadowlands needs some kind of army to protect it from outside forces and the other cosmic threats, because if someone breaks Oribos all the anima in the universe will get sucked into the Shadowlands, basically killing every titantforged in the universe. So they give the Primus the ability to see into the future a short amount of time (I don't remember if this is still canon) and teach him all kinds of magic to be able to face every threat. The Maldraxxus army is already composed of honorable warriors, so it would be hard for them to go bad (although not impossible as we saw).
- Revendreth and the Maw - There needs to be a place to imprison the most dangerous souls that could be a threat to Oribos, that would be the purpose of the Maw, but at the same time you can't allow the Maw to grow without control getting more and more souls through the ages, so you need a filter to ''redeem'' those bad souls. This is where Revendreth comes into play, it would be basically a filter for bad souls so the Maw doesn't get full of anima like it happened during the expansion.
With some tweaks here and there I think you could make it work to retcon the First Ones, maybe the titans called themselves First Ones when dealing with the Covenants to not give them too much information about themselves, something like that.