I dont think one has to be an official captain to be a leader... Shakib is not only our best player but also one if the senior players now...
On the field I'd like to see him be a bit more proactive. If anyone feels he is stepping on their toes(Mushy/Riad) he should tell them to piss off.
During the disastrous captaincy spell of Dilshan, Mahela was still very active on the pitch, field placings...talking to bowlers and at times talking to Dilshan about bowling changes and what not.
Shakib is already a leader...he leads through example...but time for him to become more involved without the annoying off field captaincy hassles(mushfiq can have the glory and blame)