I doubt Merlin will be a battle mage or any type of warrior type fighter of Galan's caliber.
I doubt she will turn into an old woman.
I doubt that her current body is the body of someone else.
I do think it's possible that she does have a "serious switch" for when she truly fights. Hopefully it deals with Aldan.
Whatever race (races?) she is, I do think it will affect her appearance when it's revealed, but something on the level of Mel's demon form or Elizabeth's Goddess form. Nothing permanent or obvious unless active.
Now, what I actually came to post.
How do ya'll feel Merlin matches up with her thematic Sin? Well? Not so well? It feels she has the least in common with her Sin. It's a case of telling, but not really showing, in my opinion, where we are told that she is this "insatiable" woman when it comes to knowledge or whatever, but the same could be said about Gowther back when he didn't have emotions. Her "Gluttony" comes across more as a great interest in the magical arts or artifacts, but nothing extreme or anything. Hell, she could easily be another Goat's Sin of Lust, in my opinion as that seems more up her alley thematically.
She doesn't have any hint of wallowing in an excess of magic or spells or wasting anything. The closest thing we can get to something like that is Infinity, which is something that could hint at an inherent excess, to the point of being bottomless, but instead of anything relating to Gluttony, the point we were mainly supposed to get is that it stops time. I want spells and magic artifacts to be spilling out of her, like she has so much she has no idea what to do with them. I want her to do useless stuff with the most powerful magic. I want her to learn magic for the sake of learning it, then discard it. Like have her learn lesser versions of the magic her comrades use just because she wanted. Have it be revealed she half learned spells due to other spells becoming an interest for her. Just relate it back to Gluttony, excess, and waste in some way.