Oh, there are many opportunities if you consider which parameters affect a diffraction pattern and what does it mean: similar. You can start with all phases containing elements which are in general hard to find with x-rays. The second group contains all phases where the corresponding elements have similar positions in periodic system like Fe, Co, Ni; T, W; Ga, As etc. There you can change the concentration from 0 to 100 and the differences are only visible if you measure carefully. Now you still have to consider experimental effects.

With free software (like Powdercell)  you can generate the theoretical patterns which enables a direct comparison.

You can do a lot with XRD (and other methods), but you always need to consider the limitations. It is much more important to know the limitation of a technique than the capabilities (which you get presented by any company for free).