As a Roblox developer, it is currently too hard to layer images on mesh parts.
Rather than deal with the longstanding hurdles of layering texture ID’s, developers would easily accept a feature for Decals and Textures to render based on the UV coordinates of a parent MeshPart rather than being projected onto one of the face normals.
Decals and Textures are already highly customizable with their Color3 and Transparency values, as well as support for alpha channels and the newer ZIndex property for layering.
While being able to layer SurfaceAppearances would be ideal for PBR effects, SurfaceAppearances themselves continue to suffer from quality compression and lack of in-game customizability.
If Roblox is able to address this issue, it would improve my development experience because it simply needs to be easier for developers (and to some extent, players) to be able to customize mesh objects in realtime with the direction Roblox is heading in with the expansion of dynamic Rthro features.
While there are workarounds to achieve layering textures like using SpecialMeshes, they are not compatible with newer features like mesh deformation.