This is purely speculation and it might never come to pass that Narendra Modi will become Prime Minister. But if he does form a government, the first problem will be his Cabinet formation.
There are about 40 people who were Ministers in Atal Bihari Vajpayee’s government of 1998- 2004 vintage still around. They will expect to become Ministers. Then there are other seniors like Kalyan Singh who will expect to be made Ministers.