I think that before DLR can ad any additional gated attrations, they must first achieve some real success with DCA. This was a huge investment that some might call a risk. I really like that park, but nothing appears to have worked there.
As a CM, I went to a "preview" of the park before it opened in 2001. It was nice and I was quite impressed. A few weeks later I returned with some of my fellow CM's over President's day weekend. The crowds were nowhere to be found. I returned again over Memorial Day, in the height of summer and on Labor Day weekend. Still, there was nothing in the way of crowds like over at DL.
DCA has not taken off yet. But I am confident that the park eventually will. I was last there this past June. Again, it was a big event (Super Soap Weekend), but the crowds were nothing impressive. I think the park will see increased attendance when TOT opens next year. Hopefully that will be the shot in the arm that they need.
DCA is not a failure. I remember DAK had a lot of issues when it first opened, but a few years later, it was a hit. As for other "gates" at DLR. Don't hold your breath. It took over 45 years for DCA to become a reality. There is not enough room on the property for another gate. They can't afford to lose any more back stage areas. A water park or smaller attraction near the property is a possibility, but i'm sure it is still several years off.