"If we architect a space system, we can starting thinking in terms of modularization and standardization. Propulsion and habitation requirements are different for short- and long-term missions, but we can define broad categories. We can apply what we already know about space vehicle cabins to design a generic short-haul habitation module for the Earth-Moon system; and what we've learned from ISS and the others to design a long-haul module for planetary missions. Then propulsion: Chemical engines for short-duration missions, maybe ion or other "advanced" technologies for long-duration. We might continue to get by with solar power in the inner solar system, but we'll need nuclear generators eventually. Fasten the habitation modules for a given mission type to a generic "keel", plug in the reactor or solar panels, and strap on as many of the type of engines required for the mass. The electronics could be as standardized as today's avionics."

I'm thinking in even broader generic terms than that. Why have differing propulsion systems? A common nuclear engine could be used to fulfill the needs of any mission in the inner solar system. The same could be applied to habitation modules, although some specialization for life support modules may be needed depending on the location they're to be setup in.

Below is a picture that gives a fairly close view of the type of ship I have in mind. The reactor and engines will be on one end with a long truss for a keel, and the habitation section on the other end. The long truss would be used to store the equipment needed for a particular mission, whether it be landing craft and modules for the Moon or Mars, sections for a Lagrange point station, extra crew modules for a deep space mission, or whatever else is required.

A few of these wouldn't hurt either. They'd come in handy for all sorts of jobs.