So, we all expected to get a trial fight with Amaterasu in 4.X, but we only got 2 of the 3 siblings. But has 5.X given us a way to finally fight her. There are two possibilities, one I though of and one my friend thought up once I told him mine.
My Idea: The Eden Raid.
Okay so in the Eden Raid we are tasked with returning the elements to The Empty, and as started in the first third we'll be using our memories of Primals to bring back an element. We know that we'll be fighting the 6 classic Primals, one for each element, but there is also speculation of fighting incarcerations of Light and Darkness, but who could represent them? Now most people speculate Odin for Darkness, but that doesn't feel right, he might be called The Dark Divinity, but that's mostly because he is dark, not because he represents it. But there is one MSQ primal who would fit Darkness perfectly, Tsukuyomi.
So, though the concept of Light and Dark can be interpreted in myriad ways, the easiest way is to just view it as night and day. We have Tsukuyomi as the God(dess) of the Moon and Divinity of the Night. So when the time comes to think of a Primal to represent Darkness, they're a show in. But what about Light. Well I could easily see Urianger casually mention that in his studies of the far east and their various kami's that he learned of Tsukuyomi's sibling, the Goddess of the Sun Amaterasu. From their we could somewhat "alter" our memories of Yotsuyu and replace parts of them with Tsuyu and for the ferocity maybe mix in some Asahi. It's not likely to happen, but it's not an impossible scenario.
Friend's Idea: MSQ related
My friend's idea was a bit different, he'd like to see Amaterasu integrated into the MSQ somehow. We dont knowhow the new batch of trials will be in 5.X yet, but this would most likely be the 5.3 Trial which has been related to the MSQ so far. In this case Amaterasu could be akin to a Lightwarden and be trying to restart the flood or something. This kinda works since The First is a "parallel" plain of existence and doesn't have to relate to the Primals of The Source as would be expected.
So what do people think? I personally doubt this would happen, but it would be interesting.