Personally, I'd like to know what happened to all of the residents of DC who evacuated, and the rest of the army. There must be something to work with in that.
I think that they could absolutely leave a "semi-closed" ending (or plot hole, whichever you want to call it). Say, you killed Adams, but one of the other aliens, jealous of Adams' power, is ready to take his place. Considering the dialogue in the credits of the first game, I can't see that being out of the question. I guess we might as well just wait and see now though, since it's really not that far away...
Im not saying that i want a complete end to the series, just a confirmed resolution example: after super metroid, every last one of the species is gone, did mean there could not be another game? No. You thought it was all over, then gave more story.
Dont let your mind wander, its too small to be out on its own Goldeneye FC: 396181417375
One of the few worthwhile details the Nintendo Power review gave us was that they were impressed by the ending, and Eric has gone on record saying that they tried hard to give it a great climactic finale, so I think we can breathe easy that we won't be smacked with another Halo-2-esque letdown.