Amaterasu is actually somewhat likely if you know the history between her and her brother, a simple change from never wanting to see him again to wanting to ruin his paradise sounds pretty fitting.
I think it's pretty unlikely though, simply because Touhou likes to keep the scale relatively low. Think about Utsuho. She's "the power of the sun" and is all super impressive and all that, but ultimately she's just borrowing a bit of power from one of Amaterasu's
servants. Touhou just doesn't deal directly with most of the
really big names in Japanese mythology, and frankly that feels intentional to me. We get a ninetailed fox, but not Tamano no Mae herself. We get the
descendent of Issun-boushi. There are some big names like Ibuki, but she's shown as an incredibly dangerous person who we're only safe from because she doesn't feel like it. Kaguya is sort of an exception, but Tsukuyomi himself (herself?) is deliberate kept in the background of the whole moon saga.
The biggest god we've met personally would Omoikane (Eirin), but she's both hiding behind a pseudonym and holding back out of respect, with the implication that she could accomplish way more if she actually had a goal other than maintaining the status quo. Having Amaterasu herself getting involved just seems like it would be way out of our protagonists' league and not a very Touhou-like story. Recall that Yorihime is considered ridiculously overpowered, and her power is just to briefly channel the power of better gods.