I don't think its the lack of laws enforcing Indianness. No country has even been great by enforcing nationalism on its own people. Had India tried to be strong and prosperous, its citizens would have had better unifying and diverse values.
Well, Israel for instance has made military enrolment mandatory for at least a year or two. If implemented in India, this can serve as a powerful deterrent to internal terrorism. The idea behind Hindutva is still the same in this case, although not as explicitly spelled out to be "Hindutva". Inculcating pan-nationalistic sentiment among all its citizens would certainly eliminate a host of problems that the nation currently is suffering from.
Another example being Uniform civil code opposition from minorities. Technically they aren't anywhere close to being a 'minority', it's political concocted term and used generously for political purposes.