Sliding Door Replacement Detail

Hi, all,

I'm looking to replace the sliding door in my atrium bedroom. I discovered that the slab is depressed approximately one inch where the old door sill sat. I most likely want to fill that depression. Does anyone have any experience with this detail? Do I fill with mortar? With exterior-grade plywood? Or do I chip out the slab further to accommodate the new (deeper) slider? Can anyone suggest a waterproofing detail?

In advance, thanks!