Why is her eye color part of the new outfit?
There's hardly any info on it yet so I'm just going to briefly guess what might be the reason. (besides being purely aesthetic because that wouldn't be interesting)
Default Fischl eye color is green. This new outfit will change that to red. Based on my theory this could mean that her dormant Archon powers are resurfacing. I would actually say it's a little early but maybe the idea could be that the Domain draws it out temporarily in a "dream-like" space which we already explored during Perilous Trail. This could be a way to help Mona scry for her true fate as I guessed might be the reason Mona's even there.
On the other hand the red eye color could be a form of bewitchment. Maybe whoever will be responsible for the event and drawing Fischl there knows who she is and wants that power so they are controlling her. The outfit's name is Ein Immernachtstraum or An Evernight's Dream both a play in real life of Shakespeare's A Midsummer Night's Dream and a play on Fischl who comes from the Immernachtreich or Evernight Realm pretty much her Plane of Euthymia.
If that's the case there's only one party that would be interested in doing that which is the Abyss. But if they already know about her now I feel that that's a little soon.
I guess we'll see. Which idea do you like better? Do you think maybe someone like Alice is responsible for the event and brought them there so she could have Fischl unleash a little hint of her true power so that Mona will be alerted setting up the future story in my theory? Or do you think the Abyss already figured out Fischl is the one they've been looking for and are trying to control her so we need to foil the plot and save her? And in doing so we'll be setting up the future story in my theory 
Tips and Tricks from a Super Casual Genshin f2p
