Strengthen rotating field, could increased rotar induced EMF(SE2), could increased the torque, or could increased speed or slip could be reduced, would not changed, the speed much(around 4%) Considerable speed control could be achieved, by frequency control, as Ns=120f/P, or winding by changing number of poles pair.

Reduction in flux (what you have referred to as strength of stator field) will cause reduction in torque produced. If the load has a variable torque-speed requirement as in pumps and fans, then speed will reduce by reduction of flux, over a reasonable range. This is the principle of speed control in fans through 'regulators'. However, if the load has  a constant torque-speed characteristic, then the motor can get stalled due to reduction in flux. Increase in flux will cause high core losses and hence not permitted.

The torque produced is directly proportional to the flux. If you decrease the flux too much, you will not get enough torque even to start the motor. It will simply stall. 

Speed control of induction motor can be achieved by changing an input frequency or poles pair. 

The torque can be controlled by the strength of the stator rotating field.

1.The idea of Flux control is well incorporated in FOC (Flux oriented control ) frequency control of induction machines using dq transformation and power electronic devices.

2. Your idea of doing away with electronic devices is interesting. Some conjectures

(not guaranteed) may lie in  unusual winding arrangement in both stator and

rotor e.g. connecting in series different number of pole winding , Pole Amplitude

Modulation etc.

 Good luck

Cheers

As a general question, here is a general answer: The rotating flux in the air-gap has an amplitude of 3Bmax/2 and has a rotating synchronous speed of Ns=120f/P (f being the supply frequency and P being the number of poles). The speed control is therefore obtained directly by the frequency control). The strength of the rotating field in the air-gap once  in interaction with the rotor will have a direct effect on the rotor emf strength and therefore an effect on the rotor current strength as well as on the rotor flux & the motor torque strength. So the control of the strength of the rotating field will either  cause a reduced torque by reducing its amplitude therefore leading to the stalling of the motor or by causing high core losses due to increase in rotating field strength.

Increased current in rotor bar, with increased field strength of rotating field at same slip of rotor, will produced higher dreaging force on rotor bars, and would increased the torque.Torque could be reduced, with increased field, only in unstable region,of operation(slip/ torque curve ch.).