Roll out thin and rolling should be even to get it crispy.
Even black channa is added along with potatoes it will also be good if you add it.
Instead of sev you can use boondhi also.
Sometimes sooji and salt alone with some oil is used to make panipuri's.
Always make stiff pliable dough to get crispy puri's.
Potatoes can be mashed or can be kept as boiled cubes.
sometimes sprouts also can be added.