Maybe it's a crazy idea, but merging the OpenMPT codebase with existing cross-platform trackers such as MilkyTracker or SchismTracker could also be a solution. To create a "super tracker" with benefits from both codebases so to say.

There is no point in doing this. UI is tightly coupled with the backend in OpenMPT just as it is in Milky or Schism. Both trackers would need to be modified heavily to even accomodate for the features that would need to be added. It just doesn't make sense to do this, and it is certainly not less work than porting OpenMPT's UI to a new window toolkit.

I guess cross-platform means Windows, Linux, MacOS. If so, mobile versions for Android and iOS should also be considered.

I still think a tracker for touch devices needs a fundamentally different UI approach, and we shouldn't sacrifice usability and familiarty of the desktop UI for the sake of touch devices. A touch-specific UI might happen later, but we already are very much limited in what we can do in our spare time so I would say this is a zero priority at this point.

I understand & agree with your feedback.

The reason for my comment was that DefleMask released mobile versions for Android and iOS some days ago. See http://www.deflemask.com / https://twitter.com/_Delek / https://www.chiptunecafe.com/caf/2019/4/27/deflemask-mobile-released-an-interview-with-delek for more info.