I'm not sure what you mean by "terminated" actually. Do you mean it
must have its load connected when the measurement is carried out? If
so, this load which could be replaced by your suggestion of a
termination resistor, would simply constitute the input impedence of
the next stage, would it now? Hold on... are you suggesting that you
replace Zin of the next stage with its equivalent resistance in order
to eliminate any reactance present in that next stage

Paul-

The next stage has some kind of input impedance, whether it is a logic gate or
an analog amplifier. I'm saying that you may be able to adjust that impedance
for minimum ringing.

If it is a logic gate, can you add a pull-up resistor across the input? If it
is an amplifier, can you add a resistor across the input without upsetting the
bias? Trying several values, you may be able to make an improvement.

Why is a long ground lead a bad thing? Because wire has inductance. Above
some frequency it can no longer be considered a short circuit. The longer the
wire, the lower the frequency. Your ringing contains frequencies that are
multiples of 8 MHz.

In addition to keeping scope probe wires short, have you compensated the probe?
Many scopes have a square wave calibration source available for checking
calibration. Another thing you can use it for, is to adjust the probe for
minimum overshoot and rounding of the waveform. (The adjustment allows you to
make the scope's probe-to-input capacitance ratio the same as its resistance
ratio.)

Fred