Layered, starting with a thin vest, possibly mesh, for summer wear, with a few well placed pockets for flashlight, possible handheld radio, etc. Any major pocket should be under the arms so as not to interfere with shoulder straps. No pockets on the chest. The pockets would open to the inside of the vest to allow the following layers to be added.
If a pocket is provided to carry, say a handheld radio or GPS in an underarm pocket, add re-enforcing material across the shoulder and down to the pocket so the fabric itself doesn’t stretch out of shape when carrying the load.
Next layer is a Nomex fleece fabric vest for warmth which zips to and over the summer vest. This layer has zip off sleeves of the same Nomex fabric and a turn up collar. This layer is the flash fire protection.
Next layer is a leather or synthetic wind shell vest, which zips to and over the fleece fabric. Has slit hand warmer exterior pockets and zip off sleeves as well.
Overall, the modular jacket should be short so as not to sit on the back edge. All fabric layers should be thin to avoid bulkiness. All collars and cuffs should be knit/elastic for comfort. Sleeve pockets can be added to the outer layers, left or right handed. Each layer can have a Velcro patch for name tag application, etc.
Make us one and I'll beta test it here in New Mexico for you.