I agree that we'll go VASIMIR, perhaps after ion thrusters for the Earth-Moon supply deliveries, and then eventually eliminate the chemical thrusters for lunar ascent/descent and go with electromagnetic launch and recovery, if not something even stranger like rotating orbital cables and such.
But to get to that point, crewed lunar vehicles are going to have to get a lot cheaper and more frequent than the SLS can manage under any
projection, and if something like the Falcon 9H with flyback boosters works, its launch cost will drop to that of the regular Falcon 9 or even much lower, and with a high ISP alternative it could probably do lunar missions with a crew of three or four to the surface as cheaply as the current ISS resupply flights, which we know current and projected budgets can support.
One simple lunar fuel I don't think anyone has looked at is molten aluminum/LOX. If the tank and piping are pressure-fed titanium or stainless steel with heater elements and ceramic aerogel insulation, it wouldn't be a very big deal to make such a craft for lunar ascent, though it might need to run very oxygen rich to keep chamber temperatures manageable.