  We present a new study of quasi-elastic $W$ and $Z$ scattering processes in
high-energy $e^+e^-$ collisions, based on and extrapolating the low-energy
effective theory which extends the Standard Model with a 125 GeV Higgs boson.
Besides parameterizing deviations in terms of the dimension-8 operators that
arise in the effective theory, we also study simplified models of new physics
in $W/Z$ scattering in terms of scalar and tensor resonance multiplets. The
high-energy asymptotics of all models is regulated by a universal unitarization
procedure. This enables us to provide benchmark scenarios which can be
meaningfully evaluated off-shell and in exclusive event samples, and to
determine the sensitivity of an $e^+e^-$ collider to the model parameters. We
analyze the longitudinal vector boson scattering modes, where we optimize the
cuts for the fiducial cross section for different collider scenarios. Here, we
choose energy stages of 1.0, 1.4 and 3 TeV, as motivated by the extendability
of the ILC project and the staging scenario of the CLIC project.
