The idea has been in the back of my head recently, and I do have interest in making one...
BUT!
There are 4 major issues stopping me from doing so.
They are:
- The save file data format for XC DE has not been reverse engineered yet, and is simply not understood. @zestiva has done some work and identified a few sections of the save file, and posted the results in this thread, but that is only a start, as the file obviously contains heaps more data that is still unidentified. I think we need a community effort to figure out the data format and what each and every bit of it is for. We can only safely edit what we know after all. Anything else is just random guessing.
- A lot has changed over the last 2 years since I made the XC2 save editor. It was written in C# and using the "WinForms" UI toolkit to build the UI, which is very easy to make GUI applications with, but Windows -only. But now my computers don't use Windows as their main OS (instead using Linux). Therefore, any app I make would need to be cross-platform, which means using WinForms is ruled out (either that or force all non windows users, myself included, to run them using "wine" compatibility layer). I can still program in C# in my current setup, but I would need to take time to find a C# UI toolkit that is cross platform, and learn and practise how to program with it, which is doable, but would take time.
- I have other personal projects that I am in the middle of, they need to be finished first before I jump to a new one.
- When I am not working on the aforementioned projects, I am spending all my free time by... actually playing XC DE

Issues 2,3, and 4 have since been resolved, leaving only issue 1... which just so happens to be the biggest most difficult issue to solve.
That said, however, I have made some progress, and have written some code that can load and save a save file, and can understand a few parts of the file, mainly the Inventory and items, and the save thumbnail, and a few other small things, but that is all so far.
Here is a link to the code repo on GitLab for those who are curious:
https://gitlab.com/damysteryman/XCDESave
Keep in mind this code is just the core backend .dll, there is no GUI yet
As for actually solving Issue #1, we really need more people reverse-engineering and figuring out the file format, and which bits of data exist at which offsets in the file. Some people have figured out some of the format and posted the offsets they identified over in this thread:
https://gbatemp.net/threads/xenoblade-de-save-address.566720/
If you think you can contribute to figuring out the file format and identify what offsets in the file contain which bits of data, please contribute by posting them in the above linked thread.