Regional parties rule many states in India already. Each one has the potential to win 20-30 seats in a national election. Since many of them do not have a footprint in more than one state, in theory, they could all come together and try and cobble a government after the next elections if the Congress and BJP aggregate falls to less than 272 (the half-way mark).
That is easier said than done. The inherent differences and leadership aspirations will probably make it very difficult for any sort of stable government to emerge. Once again, policy-making and governance will suffer.
Of course, the alternative is that one of the Congress or BJP gets a majority on its own. That seems highly unlikely as of now. But two years is a long time in politics.