  At this time, the design of the International Linear Collider (ILC) is
optimized for e+e- collisions; the photon collider (gamma-gamma and
gamma-electron) is considered as an option. Unexpected discoveries, such as the
diphoton excess F(750 GeV) seen at the LHC, could strongly motivate the
construction of a photon collider. In order to enable the gamma-gamma collision
option, the ILC design should be compatible with it from the very beginning. In
this paper, we discuss the problem of the beam crossing angle. In the ILC
technical design, this angle is 14 mrad, which is just enough to provide enough
space for the final quadrupoles and outgoing beams. For gamma-gamma collisions,
the crossing angle must be larger because the low-energy electrons that result
from multiple Compton scattering get large disruption angles in collisions with
the opposing electron beam and some deflection in the solenoidal detector
field. For a 2E=500 GeV collider, the required crossing angle is about 25 mrad.
In this paper, we consider the factors that determine the crossing angle as
well as its minimum permissible value that does not yet cause a considerable
reduction of the gamma-gamma luminosity. It is shown that the best solution is
to increase the laser wavelength from the current 1 \mum (which is optimal for
2E=500 GeV) to 2 \mum as this makes possible achieving high gamma-gamma
luminosities at a crossing angle of 20 mrad, which is also quite comfortable
for e+e- collisions, does not cause any degradation of the e+e- luminosity and
opens the possibility for a more energetic future collider in the same tunnel
(e.g., CLIC). Moreover, the 2 \mum wavelength is optimal for a 2E = 1 TeV
collider, e.g., a possible ILC energy upgrade. Please consider this paper an
appeal to increase the ILC crossing angle from 14 to 20 mrad.
