Foremost, that’s an awfully interesting question!
Well, obviously, the advice from Sherlock to PM Modi would not necessarily be “what he should do”. Rather, it would be “who to be”.
I think it would be something like stop being politically correct all the time – stop having two faces on different topics and with different people. If he thinks something is wrong, he should voice that without worrying about the repercussion. He should be honest and diplomatic with the followers, party members, opposition and countrymen.
(Generally, he would give advices to Modi that would be too tough to follow for him—because he’s a, well, “politician”!)
We all know that Sherlock is as blunt and unapologetic in his beliefs as ever. He is brutally honest in his views and opinions. He has all the talents in the world but he when it comes to his distinct personality, he’s not humble. He holds his opinions above everyone’s. And most importantly, HE DOESN’T SPEAK JUST FOR THE SAKE OF SPEAKING; he means what he says.
So yeah, just reflecting his own personality, he would advice PM Modi on “who to be”.
P.S. Sorry, you asked for “one”, I gave many!