  The proton elastic form factor ratio is accessible in unpolarized
Rosenbluth-type experiments as well as experiments which make use of
polarization degrees of freedom. The extracted values show a distinct
discrepancy, growing with $Q^2$. Three recent experiments tested the proposed
explanation, two-photon exchange, by measuring the positron-proton to
electron-proton cross section ratio. In the results, a small two-photon
exchange effect is visible, significantly different from theoretical
calculation. Theory at larger momentum transfer remains untested. This paper
discusses the possibilities for future measurements at larger momentum
transfer.
