Your first problem is the probable size of the drain...a tub is generally plumbed with 1.5" line, and a shower requires a 2" drain. Now, since it is in the slab, and some locales have a minimum size required for below slab, it may transition to 2". But, you'd have to crack some concrete. once you've done that, you could put the drain anywhere you want and not be limited to a specific shower pan. Depending on the labor rates, and the cost of the pan, it may be about even to have a tiled shower base installed, then that could be any size or shape you wish with the drain where you prefer it.
The reason a shower is required to have a larger drain than a tub is a tub has a lot more capacity to hold water if for some reason the drain gets blocked before overflowing - plus, the tub has an overflow drain connection that a shower doesn't. Drop the wash cloth, cover the drain, and if not careful, it could be running over the curb. Not likely to happen in a tub.
A good resource for tiling help is www.johnbridge.com. Even if it is just to understand if your contractor is building it right, it's a good resource for knowledge and understanding.