GNUstep is a mature free and open-source Framework, suited both for advanced GUI desktop applications as well as server applications. The framework closely follows Apple's Cocoa (formerly NeXT's OpenStep) APIs but is portable to a variety of platforms and architectures. [https://www.gnustep.org https://www.gnustep.org]
Applications written for GNUstep also run on Mac with a "natively"-looking interface. They also run on Linux and have look and feel consistent with other GNUstep applications.
GNU Octave currently has a Qt interface. This project is dedicated to writing a GNUstep interface for GNU Octave.
'''Required skills''': Knowledge of Objective-C, Cocoa, GNU Octave codebase
'''Difficulty''': difficult (large volume of work)