Is there a motor design where a rotor gradually moves into a stater?
There IS such a motor design, but not for the purpose you are thinking of. In the crane industry, a company named Demag makes a conical rotor motor so that the act of energizing the motor creates axial (lateral along the axis) movement so that the motor disengages a mechanical brake assembly on the end of the shaft. This avoids needing to have a separate solenoid coil for the brake.



If you want to experiment, you could buy one of these and remove the brake assembly, then rig up a way to move the rotor in and out of the stator. But honestly what you will likely learn is what Janka has already told you...

Another variant of motor designs, no longer used, was the "super-synchronous" motor, in which the stator and rotor could both spin freely. So at start-up, the STAOR would spin, because the rotor had the load connected to it. Then when the stator got to speed, an operator would tighten a band around the stator to stop it slowly, which would make it transfer its mechanical power to the rotor. If you used that design as a generator and rigged a way to only partially clamp the stator, you could control the output. It's hard to envision with words, so here's a cool video of one in action:
https://youtu.be/fNuI6keQXYA (I call this "motor porn"...)