When approaching the implementation of 3D geometric transformations like vertex rotation and scaling, it is essential to understand their foundational role in any graphical or modeling application. Completing these features lays the groundwork for more complex functionalities. Once they are in place, you might look into these subsequent tasks:
Enhancing User Interface Controls: Post-implementation of basic transformations, focus on UI elements that will allow users to interact with these features seamlessly. Sliders, buttons, and more intuitive gestures can enhance user experience.
Implementing Undo/Redo Functionality: Allowing users to revert changes or reapply transformations enhances usability, especially when dealing with complex models or animations.
Integrating Texture Mapping and Lighting: After mastering transformations, bring models to life by adding textures and simulating realistic lighting, which will require understanding of normal vectors and shading models.
Optimizing Rendering Performance: Ensure that your transformation algorithms are optimized for performance. You might need to explore spatial data structures or level-of-detail techniques for handling complex scenes efficiently.
Adding Animation Support: With scaling and rotation implemented, explore keyframe animation or skeletal animation systems to bring dynamic movement to your models.
Collision Detection and Physics: If your application involves interaction between objects, look into basic collision detection algorithms and consider integrating a physics engine for realistic interactions.

Ultimately, the choice of features to work on next should align with the goals of your project and the needs of your users. If you have a roadmap or product vision, use it to guide the prioritization of these subsequent tasks.