i'm replaying hw patch and just got through the wod arc and it got me thinking about a little au where ardbert doesn't go back to the first with the rest of his comrades. that's it, that's the au.
but in all seriousness and further elaboration, nyelbert says the power of their crystals is almost gone but then ardbert spends a century just vibing as a half-faded shade, wandering around the first. if he had enough strength left to exist like that for a century, i feel like he could handle a few years on the source.
the word of the mother wouldn't let him help with the flood anyway, like... she took him home, but at what cost? there's not really anything that changes by him staying on the source other than where he is and what he does during that time between hw patch and shb.
so my thought is basically her saying "i'll take the rest but fate's not done with you yet. don't worry, you will see the first again," and then leaving him there, where he proceeds to join up with the wol and becomes a (temporary) scion.
then, come stb patch when the exarch is doing his gacha pulls, he pulls ardbert. of course, because ardbert has already (technically) given up his physical body (and exists in the same "physical" way the scions do on the first as souls), he gets completely yoinked, whereas the scions only have their souls taken.
the neat thing about this is that shb can literally play out the exact same, too. he can still merge with wol (unless....) but this way he actually gets to physically help defend the first in the meantime. he can help around the crystarium, he can help fend off sin eater attacks, he can continue to travel with the wol as they slay lightwardens. he can right some of his wrongs and then play his part in the story's climax.
he'd probably have to keep a low profile, too. maybe keep his really dumb alias arbert while going around the crystarium, because the first's wols are infamous.
...of course, they're also supposed to be dead, so maybe he just uses his real name and people give him a look like "damn your parents must have hated you to name you after the warrior of light lmfao"
i like the idea of him and cylva meeting again, too. she probably never expected to see him again, but there he is. and then they rekindle their friendship, bonding over the both of them being the ascians' puppet fools because that's!! the kind of person he is!!! ("he chose mercy. he chose love, and i was undone"!!!!)
i think he'd want to help her restore the thirteenth, too. he'd want to send her home. i think if he didn't rejoin with the wol in shb (and stayed in the first), that would be something he'd want to help her do.
so like... the paths his canon could take are basically
full game canon compliance. he goes home in hw, shb happens as-is
he stays on the source in hw, helps with shb, rejoins at the end.
he stays on the source in hw, helps with shb, doesn't rejoin, stays on the first, goes on to become part of the void quests/void story.
he stays on the source in hw, helps with shb, doesn't rejoin, decides now that his world is safe there isn't really much there for him so he opts to travel with the wol, even if that means leaving the first.