The BJP has indeed come a long way, from being a so-called upper caste, North Indian party, it has now captured the largest state in the Northeast with a Chief Minister who is a tribal, in the process displacing a once-invincible political party. Tarun Gogoi, who decided to go it alone and project his son (who is another one in Rahul Gandhi's coterie), clearly suffered a huge setback, not just from a strong BJP with a local face, but also by the defection of Himanta Biswas Sarma, who was the master strategist for the Congress and whose loss the Grand Old Party could not possible deal with. Sarma did not mince words in an interview to NDTV, when he denounced Rahul Gandhi and the entrenched dynasties of the Congress. And while some surmise that a tie-up with the AUDF could've helped the Congress, the fact is that the overt communal tone of the former would've further polarized the electorate, which was already seething with rage over Gogoi's open-door policy for illegal Muslim immigrants.
The BJP has also learned its lessons from the annus horribilis that was 2015. It gave great powers to the local leadership, projected a single leader who was very popular in the state, used the Central leadership on as needed, and stuck to its core issues: deporting (or at least disenfranchising) illegal immigrants and rebuilding a wrecked economy. And although the victory seemed perfect, even destined, the truth is that it takes a lot of effort for a party as massive as the BJP to change tactics so swiftly, which is certainly to the credit of Amit Shah.
Now the BJP is in government in Assam, Nagaland (as the junior partner in the DAN), and Arunachal Pradesh (with Congress rebels who brought down the party's government in that state, the Congress' only victory in 2014), while also having an understanding the SDF in Sikkim - that's half the states in the region, and a majority of the population (although Assam takes care of that by itself). Amit Shah clearly has his eyes set on the rest of the region, and Sarma appears to be his go-to man there. Some interesting times are ahead in India's Northeast.