The waveform can't be rectangular, for several reasons.
Limited switching speed due to device capacitances, inductances and series resistances is the most serious limitation.
It would also need a rectangular gate waveform.
The output power in CW operation can't be determined from maximum ratings. You need some safety margin for Vds
and have to consider rds voltage drop. The achievable AC output voltage is below Vds/2. As said, the transistor is
specified for 15 W in linear (class AB) operation. As a guess, Class C output at 200 MHz can be possibly higher, but
hardly more than 50 W.
P.S.: In a real design, you have as input signal either an "analog" sine or a "digital" signal carried by a fast I/O
standard, e.g. LVDS. The latter possibly has rise-/fall times down to 50 or 100 ps. But how do you drive it to a
FET gate? That's probably the most serious issue of your "rectangular wave" design idea. The other is at the
output port: Rectangular wave implies dI/dt values and respective inductive voltage drops, that easily exceed
any available transistor voltage rating when going to multiple 100 MHz.
Class C in contrast means: (almost) easy sine drive waveforms, as much switching speed as the device can
handle, existing device capacitances and inductances are treated as part of the impedance matching network.
Technology advances, however. Today, switching ("rectangular wave") amplifiers are common e.g. for 13.56 MHz
ISM applications. Possibly they extend to 100 MHz or more one day.