Something like this? (Hope it posts.)

Choose a tile for your shower floor that is darker as above example (use the colors of the glass tile strip, but tile the floor and run it up the wall as high as needed to meet the original tile. So the whole thing looks "planned" not "patched:".

I really like the idea of tying it in with the floor.

The bathroom is small, I would do a barrier free shower with a nice linear drain. Use 10mm frameless glass doors that utilize no bottom track. It will make the bathroom feel bigger...otherwise you will just replace a 30" tub with a 30" shower floor minus the curb, Then it will feel really small.