What will happen if rotor diameter is increased in an induction motor?
Well, the rotor diameter will increase... and with that the airgap, as mentioned in the comments, will reduce.
How the torque changes ?
Not much. The flux in the airgap is determined by the stator voltages, and those are not modified.
But the leakage flux in the airgap will reduce a bit and therefore a slightly better coupling between rotor currents and stator flux will be obtained, leading to a slightly higher torque. A second order effect though.
How the pf will change ?
It will increase because the energy in the airgap, which indeed has a lower volume now, will be reduced and with that the magnetisation current, which causes the power factor to be < 1. First order effect.
What will be the disadvantages?
More friction in the airgap, worse cooling, larger torque fluctuations when the rotor grooves pass the stator grooves and as a result higher forces on the windings, higher accuracy requirements, stronger axis needed to better prevent bending, tighter bearing monitoring required to prevent rotor crashing into the stator.