Why is Luffy's smile so different in his latest wanted poster? I don't remember which chapter this started in, but I've noticed that his smile is not as wide as usual.
It's very different from his wide, rubber-skinned smile. Now it looks more like a girl's smile in One Piece.

I know it doesn't matter much but could there be another artist besides Oda who is working on One Piece? Or could this hint to something related to a change in his stretchiness or something? 

I've also noticed that he looks younger than pre-timeskip. Both of these could simply be a change in Oda's art style though.



The last poster was shown in the manga too.
I actually think it's simply the evolution of his art style. Your theory is tight and even though Oda has been very careful with his little hints, I don't believe that it is actually the case. Truth be told, I have not noticed any changes in his smile even in the latest chapters. As cheerful as always