So, the first part of this is going to be the annoying nerd part where I talk about a potential loophole in Oma's second rule: She cannot modify a character in any way, shape, or form. Full stop.
however
Emphasis is placed upon this being a direct act. So, ergo, she cannot turn someone from a kobold to a wyld elf directly... But it's not theoretically impossible for her to create something that, when consumed by a kobold, would turn them into a wyld elf as part of a one-way trip. This might get slapped down with a quick "Hard No. I'm not micromanaging this headache" which is totally fine since this is all a hypothetical, but this is the loophole that the rest of the argument is based on.
Getting into the meat of this particular chain of thought, there exists one party member who is functionally immortal and absolutely hates it: Agni. It's also fairly sternly implied that she, at one point, made contact with Oma or another geine and wished for her current immortality. It's also well known that, by this point, Agni is rather unhappy with that (theoretical) wish. Thus, she stops the champion from making the same wish that she (theoretically) did and now regrets. The reasons she gives you for interrupting this wish are solid: Watching your friends, family, places you've been, places you like, and all the world change and evolve and wither and decay kinda sucks. Being the only thing left standing out of what was once something of immaculate beauty is certainly something that, when repeated enough times, I can understand why Agni would come to warn anyone away from making this same decision.
However: What if we were to impress upon her that she would no longer be alone? A lot of her regret is buried in watching things die again and again and having to leave everything behind again and again. But what if she had a companion that was as persistent as she was? What if we assuaged her worries and regrets by making an oath that we'd be with her, every step of the way. She'd have someone who would understand her on a level that most people fundamentally can't. Someone who's in the same boat that she is. Someone that she could finally spend the fathomless depths of time with, forever.
Theoretically speaking we wouldn't need to change her stepping in to reject the wish initially, just adding things after it. It could even be made a pseudo dialogue boss-fight like a few other encounters are. Something where you have to actually hear Agni out and understand why she doesn't like being immortal anymore, and then carefully work your way around to explaining that you want to be like her because you want to be with her, unto the end of time. Could even have it be arranged where if you're dismissive towards her, or phrase your arguments around how cool it would be to live forever, etc, and Agni just up and leaves and you lose her as a companion, with her deciding that as long as you're chasing immortality against her wishes then she's not going to be there to watch you make the same bad decisions she has.
The finale of this would be her agreeing to let you make the wish, on the condition that you don't just wish for immortality: You wish to be an immortal phoenix, just like she is. That way, due to how rare they are, any rumors that crop up of a phoenix popping up in a location will be a beacon for her to come looking to see if it's you. Mainly because since she agreed to it, she's not letting you out of her sight for longer than she's comfortable. That was the deal, after all: Together forever, whether you like it or not.
However, as it would be pointed out: This now breaks Oma's second rule. She cannot modify the body of a person. Thus the creation of a one-way transformation item, the pseudo soulbond: The Ember of Eternity. (Or, in silly mode: Phoenix Down x999, unless that's a risky gamble due to copyright issues.), it would follow the same rules as a soulbond transformation: Unique icon, you cannot change into anything else, and you cannot go back. Plus it would make Kasyrra pretty angry. But how would this item be created?
Well, Agni would need to step up and donate a phoenix feather. Highly coveted by all. But as we've seen with multiple other transformatives in this game: a lot of them are only skin deep. To be a proper phoenix, just like Agni, is going to take a bit more than that. That's where Oma and the rules lawyering all the way at the start comes in: While she cannot alter your body she could, in theory, create a transformative item that is potent enough to cause you to be entirely reborn through it's use. And that's what she would infuse into the phoenix feather in order to create The Ember of Eternity. But this item would come with a stern warning: "There. It is done. Upon infusion into your body this ember will ignite your soul, beginning the transformation into an immortal phoenix. But before you accept it, I need you to know two things: The first is that once this begins, there is no way to stop it, and no way to turn back. The second is that this will be one of the hardest tests of willpower you will ever face. To be reborn as a phoenix is not a kind thing to either the body or the soul. It will be an agony that you will likely never forget, and if your will faulters for even a moment during the transformation: You will be consumed. Gone. Reduced to ashes. And not even the most powerful magic in the world will be able to bring you back."
The entire transformation would be a lengthy series of scenes describing the reshaping of your body and soul, filling the dialogue box with maximum minus one options for "No" or "Stop", with one option for "Endure" floating around, requiring careful reading to make sure that you don't trick yourself into a bad-end and burn to a crisp. Naturally once it's done you'd get the chance to examine yourself, and at that point you'd get to pick through your final appearance customization, much like with the other soulbond transformations in the game.
Then, at last, you'd get a handful of new fire-based powers and Agni could potentially teach you a few tricks of hers. The main notable one that I can think of is that, as a passive power, you'd get a once-per-day self-only revive, basically just a trimmed down version of the white mage's ultimate power, giving a mechanical significance to being a phoenix.
To repeat it at the end of all of this: This is probably never going to happen, but I had this thought come to mind and I figured it'd be neat to write it down somewhere instead of just shrugging and throwing it into the void.
TLDR: It would be cool to be able to get a one-way phoenix transformation by beating Agni in a dialogue boss-fight and then having Oma use a rule-loophole to make an item capable of transforming you completely into a phoenix as part of a one-way pseudo-soulbound trip.