Is your feature request related to a problem? Please describe.
So Optifine, one of the best Shader client mods with one problem: Performance
and as we all know Iris aims to solve that problem, but so far, It has way less features than Optifine has.
Optifine has a feature for Vanilla Shaders, to make some textures be able to have a glowing effect, an effect like Enderman's eyes, but for blocks, the fire in the furnace has a glow, separate than the rest of the texture. it also adds support with it for resource packs and mods.

Describe the solution you'd like
A clear and concise description of what you want to happen.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

What ARE Emissive Textures Exactly?
Emissive textures are overlay textures that glow in the dark. They can be added to basically any texture using Optifine. Resource pack creators can choose to make an entire texture emissive, or just elements of a texture.

Iron Ore Files in assets\minecraft\textures

iron_ore_glow.png

iron_ore_glow.png.mcmeta

In Game

emissive.properties (don't know what this is for)

Most shaderpacks that use labPBR already define an emissive texture as laid out in that standard; so having an entirely separate texture for it is superfluous.

This is out of scope for iris, sadly.

This is out of scope for iris, sadly.

The emissives format seen in the third screenshot is the Chisel/CTM format, which is provided by CTMF on Fabric. This mod is compatible with Iris. The last screenshot shows the emissive file name configuration for Optifine format emissives, but it is a different format so this file is not used by CTMF. There is no Fabric mod for Optifine style emissives yet, but I may make one eventually.

That's Complementary's integrated PBR and emissives (through code)

using shaders you can do this