IF the ground and water table are stable, you could just do a mudbed on what you have. Having that area recessed would allow you to make a barrier free entrance (i.e., no curb) if you wanted to. But, to be strong enough, you'd want the thinnest portion to be at least 1.5" thick and ideally, have welded reiforment 1/2-way through the thing. Some areas are notorious for not using a liner, and just letting any moisture that gets beneath the tile just drain into the earth...that is NOT a great idea!
If you go with a conventional shower build, you have three layers before you get to the tile:
- preslope
- waterproof liner
- setting bed
Then, you can add the thinset mortar and set the tile on it. The setting bed is the same thickness all the way across to follow the preslope with the liner on top of it...because it follows the preslope, it needs to be parallel to it, and thus, will be the same thickness.
More modern alternatives to a shower build use surface waterproofing that you can tile directly to. Those would only require the preslope...then, you'd add the waterproofing layer that gets bonded to the preslope, and it also accepts tile applied directly to the top of it. That type would normally require changing the drain to on designed for surface waterproofing, but there are techniques to reuse the old one. To be honest, at least with the picture angles supplied, it looks more like a floor drain rather than a shower clamping drain. If so, then that is NOT a proper drain for a shower that meets any national standards!
Many people think that the tile makes the floor and walls waterproof...it does NOT! Plumbing code requires a shower pan to be waterproof, and you cannot achieve that with only tile...there MUST be a waterproofing layer beneath it. And, because at least some water will penetrate, there must be some way for water to percolate through the setting bed and drain beneath the surface - with a clamping drain installed properly, there are weep holes that allow that moisture to escape. It looks like they didn't do any of that, and just let the small amount of water drain into the soil. That can draw ants, termites, and tree roots.