I’d love to see support for channel-packed textures in the importer so multiple maps can be combined into one file, along with a multi-tint system available in the object edit window and avatar appearance editor. I hope it's okay to put two suggestions in one (never posted on this feedback thing before), since I sort of hope for them to work in tandem.
RGBA Channel-Packed Textures:
Allow the texture importer to pack maps (e.g., normal, emissive, ambient occlusion, etc. - or - channel information to specify "tinting zones") into the Red, Green, Blue, and Alpha channels of a single image.
Multi-Tint Support:
Add the ability to apply up to three color tints (based on the packed RGB channel information) within the object edit window and avatar appearance editor.
Packing maps into a single texture reduces uploads and texture bloat, which improves performance for everyone. Multi-tinting would make it easy to offer better customization options directly in-world without needing as many separate textures; giving both creators and users more creative flexibility with less complexity.