  One of the prime tools to search for new light bosons interacting very weakly
with photons -- prominent examples are axions, axion-like particles and extra
``hidden'' U(1) gauge bosons -- are light-shining-through-a-wall (LSW)
experiments. With the current generation of these experiments finishing data
taking it is time to plan for the next and search for an optimal setup. The
main challenges are clear: on the one hand we want to improve the sensitivity
towards smaller couplings, on the other hand we also want to increase the mass
range to which the experiments are sensitive. Our main example are axion(-like
particle)s but we also discuss implications for other WISPs (weakly interacting
slim particles) such as hidden U(1) gauge bosons. To improve the sensitivity
for axions towards smaller couplings one can use multiple magnets to increase
the length of the interaction region. However, naively the price to pay is that
the mass range is limited to smaller masses. We discuss how one can optimize
the arrangement of magnets (both in field direction as well as allowing for
possible gaps in between) to ameliorate this problem. Moreover, future
experiments will include resonant, high quality optical cavities in both the
production and the regeneration region. To achieve the necessary high quality
of the cavities we need to avoid too high diffraction losses. This leads to
minimum requirements on the diameter of the laser beam and therefore on the
aperture of the cavity. We investigate what can be achieved with currently
available magnets and desirable features for future ones.
