a) Beam dynamics limitations from increasing the current (space charge limits, halo development increased H-stripping)
b) Equipment upgrades for reliable operations (e.g. klystrons/modulators, cryogenic cooling capacity)
c) SRF cavity issues: structure improvements, conditioning, in-situ repairs, retrofits, operational experience/margins with existing SRF cavities, Lorentz-force detuning compensation
d) Power coupler limitations