Ability to digitally (cryptographically) sign documents
While the handwritten signature is good, better would be an ability to digitally sign using the private key of a public/private key pair. This would be used together with the handwritten signature, so you would first use the handwritten signature, then flatten, then digitally sign (it would be helpful if this could be set up as a "one-step" process, subject to needing to enter the passphrase for the public key.
It would also be nice to be able to save the decrypted private key, and be able to digitally sign using a fingerprint, on devices with a fingerprint scanner (to save time required to type the passphrase). This would require a warning the user, about the security of doing it, especially on a rooted device.
By the way, I have used Xodo in court a few times over the past few days in Multi-Window mode on a Note Pro 12.2, with LectureNotes on the right hand side for my handwritten notes of the proceedings. Multi window together with the tabs is proving extremely convenient as I can do my part without needing paper, except what I need to give the court and the other parties.