Same for me… Ever since TB Sanji is treated like a joke character.
But even if it is just a "joke" now it may turn out to be important later on
Well i think that Oda giving the rest of the crew some adventure off screen and with Sanji as their leader with a special bounty must have something to do with developing his character and give him more importance to the story. I do not think that Sanji will progress more as a fighter although he has a great potential as a physical fighter and his Diable Jambe. Luffy will always be the main physical fighter but Sanji is the most complex character from the crew and that can be his advantage.