Personally talking I don't need that Delphi make visual apps on Linux, only services or web apps, the actual model that use PAServer used to deploy and test executables on others platforms I think is viable to be used on Linux to.
A UniGui app is in his interior a VCL app.
The problem could be the inner of components. We know that internally a UniGui component use a GDI handler that is a Window only feature, thas minds that use calls to windows api to work
Is possible that Delphi use internally a Wine package to work to. The problem with this is that Wine does not support windows services only Desktop and console applications.
This is the problem that forces to Borland times ago to use QTWidgets to implement Kylix. I don't know how FireMonkey implements internally but it work on Android an IOS.
Then UniGui could migrate to FireMoneky internally to be capable to be used as a LInux app.