One of those that should be done immediately is bring back development of the SM-3 Block IIB. It builds on the proven SM-3 base, and it *would* be able to intercept ICBM class threats. Launched from ships, SM-3s can be stationed nearly anywhere without need of permission or tricky/fickle basing rights.
They also need to build up and deploy THAAD - that is a very capable missile. Finally, we need to get back into going after threats in the boost phase - midcourse and terminal systems are a good idea and demonstrably within our current technological abilities. Boost phase is (would be) even better. Gives us a longer engagement envelope, and puts the problems over enemy territory, not ours. The airborne laser was a great research tool, time to take it to the next level.
Other missile defense systems, including Aegis, THAAD, and Patriot, are generally classified as “regional” systems, and are geared toward short to intermediate range ballistic missile threats. While some may have homeland defense applications in certain circumstances, they have much smaller coverage areas as compared to GMD, and generally much less capability, if any, against ICBMs.
Conversely, GMD is not capable of shorter range, regional defense missions. North Korea’s short and medium range missiles threatening South Korea and Japan, for example, fall outside of GMD’s engagement envelope. These threats require other solutions, such as Aegis, THAAD, or Patriot.