Monolith's project is a possibility of being revealed at E3, although I wonder it won't be announced until much later. That said, after the development of Xenoblade 2 in late 2017, I'm sure Monolith immediately went to work on their new project, and also at around the same time, they were likely working on XC: Definitive Edition, plus BOTW2 with Nintendo.
Looking into it further, Definitive Edition was announced in Sept. 2019, and launched in May of 2020, and I want to say Monolith said they worked on it for 2 years, so that would align with starting development after X2. BOTW was finished of course in early 2017, and its sequel started I'm sure right after that. So as a timeline, some of their staff started on BOTW2 after March 2017, followed by another set of staff working on XCDE in Late 2017, and then there's another team working on a new project. After DE was launched, that means that team could've been split between their new project, and BOTW2. It is also possible they're setting aside a team to launch a Definitive Edition of Chronicles X for Switch.
Based on this, this is what I how I think their staff are split up (A meaning top priority, and C lowest priority):
A-Team: BOTW2
B-Team: New Project
C-Team: Xenoblade Chronicles X
Monolith Soft is clearly a pretty big development studio, so the idea of them working on multiple projects, compared to another 1st party studio that may reside in Austin, TX, is not far-fetched. If XCX for Switch does exist, then I believe their new Project is a direct sequel to Chronicles X, which I'll refer to as Chronicles Y.
BOTW2 is likely to be a 2022 launch at this point, which could mean a possible Xenoblade Chronicles Y in late 2022, or sometime in 2023, and of course a Switch port of Chronicles X in late 2021 as a tie-in/catch-up for those who never played it.
There may also be a fourth team that is responsible for converting many of their previous titles such as Xenogears, and Xenosaga, although I am unsure if Square owns Xenogears, or if it belongs to Tetsuya Takahashi. Xenosaga would be much easier since it IS a Monolith Soft IP. Knowing this though, maybe a sequel to Xenosaga, or Xenogears is in the works, which would also possible mean a Switch conversion of either of those titles.
I don't think Monolith Soft are working on a non-Xeno type game outside of Zelda with Nintendo, so something "Xeno" related I think is what they're ultimately working on besides BOTW2.