Must have

• import/export DDS
• automatic mipmap generation
• basic texture compression (S3TC aka DXT1/3/5 aka BC1/2/3, maybe libsquish (MIT)?)
• there is an official Microsoft's library for DDS support DirectXTex (MIT)

Should have

• authoring/import/export of cubemaps (6 separate sides)
• authoring/import/export of 3D textures (essentially 2D layers with interpolation, 3rd dimension is often space or time)
• authoring/import/export of array textures (array of independent same-sized 2D textures)
• better texture compression
  • mobile compression formats:
    • ETC2 (not sure if suitable open implementations are available)
  • desktop compression formats:
    • BC4/5 (aka , 1- and 2-channel images, using the alpha encoding from BC3)
    • BC7 (aka BPTC, much better quality RGB compression, suggested bc7enc16.c (MIT))

Could have

• mobile compression formats:
  • PVRTC2 (PoverVR-specific so less common, implementations ???)
  • ASTC (best compression ratio, legal and technical accessibility ???)
• desktop compression formats:
  • BC6h (HDR! half-float format)
• cubemap filtering (for image-based shading, see github.com/dariomanesku/cmft)
• conversion between environment map representations (see image in cmft readme)

Stretch goals

• normal/bump/occlusion map generation
• support for KTX file formal https://www.khronos.org/opengles/sdk/tools/KTX/file_format_spec/
• basic preview for material textures based on the PBRT shading model used in glTF