I wrote this architecture proposal, FLEX, a few years ago. It analyzes NASA’s approach that the ESAS study picked and notices how most of the mass in a lunar exploration stack in LEO is actually liquid oxygen. By using a propellant depot, the LOX can be lifted with tankers and any launchers imaginable (I wouldn’t use a Pegasus though). The rest of the stack is also naturally divided into about 20 ton chunks: EDS with its hydrogen, the CEV crew vehicle (Orion) and the LSAM lander (Altair).
No new heavy lifters need to be developed, there is enough US, nevermind world launch capability to support a moon exploration program. Launchers can also be improved on the run, because they are not tied to the single use, nor is the use dependant on the single launcher, and because they can fly often, hence improvements are worth the investment. This all could be achieved much sooner and cheaper than the current approach, and is much more robust for the future.
Go read it if you haven’t.
There are some comments at an old Nasaspaceflight.com thread that deal with a lot of the common questions about it.
I really don’t have the faintest idea of the background knowledge level of the readership here so I don’t know how much basics I should give, so feel free to ask in the comments if anything is unclear.