While an open lander is possible, I predict it will not happen soon. If you expect to stay at all, you need a habitat, and at least initially, it wouldn't be permanent, so why not make the lander the hab?. Later if we really have the resources to support a base we probably can support a pressurized lander, most likely even larger than the LM. I predict The next will be 3 to 4 times the payload capacity of the original, with conversion to cryogenic fuels when and if ice deposits can be utilized. Large boosters can be derived from STS and Delta IV hardware for a reasonable development cost.