What happens to a high power inductive motor if you decrease the RPM to half the rated in terms of current, torque and heating?
Induction motors are available with power ratings ranging from about 1/500 Hp (1.5 W) to 30,000 Hp (22MW). It is difficult to say just where in that range the motor in question may be. It is likely that the question is about a low to medium power motor rather than a motor that is more than a meter in diameter.

If the motor is controlled by a reasonably good variable frequency drive (VFD) it can be operated down to practically zero speed at 100% rated torque.

The motor current will not exceed the rated full-load current. The VFD input current will be proportional to the mechanical power output of the motor, so it will decline as the motor speed declines. The mechanical power output of the motor is proportional to load torque multiplied by speed. There will be some harmonic current content in the VFD input current. That will increase the current somewhat.

The heat lost in the motor will decline a little bit as speed declines, but the ability of the motor to dissipate losses will decline due to the reduction of the speed of the motor's self-cooling fan. That will limit the time that it is safe to operate the motor at rated torque and any given speed. At half speed, most motors will probably be able to operate at rated torque continuously. If the motor is cooled by a separate blower that is selected and furnished as part of the motor by the manufacturer, it is possible to get a motor that operates continuously at or near zero speed.