What main course Indian cuisine I can make out of potato with sweetness in nature?
Use them with sago to make microwave sabudana khichadi. The dish is a nice blend of flavors and one of the ingredients is sugar. Just reduce the amount of sugar to 1 teaspoon or less, depending on how sweet the potatoes are, instead of the stated 1 tablespoon. 

My experience with that particular recipe, by the way, is that the given 2 minutes total of microwave time is too little. I have a recent model microwave with 1100 watts of power but it still takes three minutes microwaving on high power + five minutes standing time for the sago to be cooked. I also prefer to use dalia (roasted chana dal / bengal gram) rather than peanuts, although peanuts are traditional. 

As an aside, sabudana is sometimes erroneously called "tapioca". The Wikipedia page on sabudana notwithstanding, sago and tapioca are not the same thing. Sago is made from the pith of the stems of sago palms, tapioca from cassava root. They're both starches and the pearls look more or less identical, but they behave differently. For one thing, sago takes about an hour of soaking; tapioca, about four or five at minimum. Soak sago that long and it will fall apart.