We have installed niches in all of our renovated showers and wouldn't be without them - they are so useful. Ours are all about 3 inches in depth. If you use a piece of stone, corian, etc as the bottom shelf that will give you that extra depth and shouldn't look out of place.

I'm not a niche fan. Too much work for too little reward. Nothing beats a well drained easy-to-clean solid surface shelf. Plus you can't screw up your tile lines.

Cut a shoe box to 2 inches deep. See if your shampoo bottles and soap will fit. Or do you want to buy special bottles that will fit?