Model viewer with basic editing capabilities.
Slice3D app that simply shows a model on-screen, and instead of GUI may even simply have a text input console to receive commands to change the view projection, adjust lighting parameters, or load a different model.
Next, add some new commands to do things like enable/disable wireframe & vertex dots, show vertex IDs, edit vertices, and save the model. GUI equivalents to these commands may be easier to use but more time consuming to implement so they can be added later.