It's somewhat ironic that post-independence India never seriously tried to introduce an indigenous national language, mostly because of its vast diversity. However, unlike former African colonies, India has some degree of pre-colonial collective identity. Therefore, your challenge is to have India adopt a national lingua franca other than English, while its regional language exist as regional languages. Your best bet would probably be a vernacularized version of Sanskrit, a parallel to China's Putonghua, which is a vernacularized version of Classical Chinese.