So I've just come across something that proposes a fun idea. What if Mystara, the entity called upon by beggars in Oblivion, was Ithelia? The idea then goes on to suggest that "Mystara" is simply the name given to a deity that some Ayleids worshipped, but no one else did. Meanwhile, beggars call upon this entity because she understands what it's like to be forgotten and cast aside.
We know nothing about Mystara, so it's a fun idea. However, it's given me my own idea. What if Mystara is Ithelia's version of Maddy? It could be such a fun way to use a deity that's only mentioned once. Also, I saw this really cool Ithelia design someone made, and it could be a fun basis for Mystara's appearance.
Plus, imagine Mora's reaction. XD
I had to look this up because I had completely blanked on Mystara and no wonder because you're right, there is literally nothing!! You could do whatever you wanted with that name, this is essentially nothing but a name thrown out by beggars in Cyrodiil and the Shivering Isles. And ngl I kinda love tying it to Ithelia.
In canon, I would think that Ithelia took the core of her mantle with her so Mystara might not have been able to fully supplant her the way Maddy tried to with Sheogorath, but the power she shed before leaving could pull together as it tried to seek her out before coalescing into Mystara whose memories of Ithelia are also broken and fragmented, and she's working with shards and pieces rather than the full picture. She can't even gather Ithelia's memories to help herself either because Mora wiped them away, so she's literally starting out from scratch with a name that means nothing and a longing for something that doesn't exist.
Maybe she starts out less as a Prince of Paths and more as a unique Daedra lord or some kind of demiprince that gives Mora a heart attack when he finds out about her.
(and while I know that design isn't meant to be pixie sized, it reminds me so much of a pixie with the wings that I'm imagining her being the size of a nixad. She's just a little guy, you can underestimate her, she definitely can't erase you from existence with a thought).