Total Commander

One big feature in my opinion that will be great, is the ability to read and write to LUKS container. As I understand the Android kernel has the necessary encryption algorithms to do that.
Probably a big undertaking, so if you ever have time it would be great if you add it. Probably best if it's an add-on for rooted devices(Don't think it'll work on non-rooted ones)

Unfortunately there are a lot of different encrypted container standards, see e.g. this page:
https://alternativeto.net/software/truecrypt/?platform=android

I was thinking of using cryptsetup which is open source and has all the support for different container standards.
https://gitlab.com/cryptsetup/cryptsetup
People in the past had compiled a binary for Android 5.0 and used command line to execute it.
There's a lot of abandoned projects that had it working but is left unmaintained for years, like guardian project
https://github.com/guardianproject/luks/wiki
But I'm not an Android programmer so maybe it's really difficult to make sure it works properly and that's why it got abandoned in first place

Unfortunately this project is licensed under the GNU General Public License v2.0. This means that I cannot use it from a closed source app. It would work if it were licensed under LGPL.