CREF my prev comments yonks ago about the force model construct of a USN/USMC ARG/MEU
I believe a TAKE would be a good fit as the US uses them to support MEUs, afloat support and prepositioned forces, they could supplement both our AORs and amphibs. Then the is the ESD, or even ESB options, cheaper to buy and own than an additional LHD, better or more suitable at some things worse at others.
Aside from these phat options there are various small/medium LST/LPD/LSD options that could complement the existing big ships. Looking outside the square we could buy/build anything from a straight LCH replacement, an LSV, something like Singapore's Endurance Class, a small Enforcer type, through to something like Japans Osiumi that is basically a through deck LSD that is mostly dock, a short vehicle deck topped with what looks like a flight deck but is actually an exposed vehicle deck that can operate helicopters.