I believe I have written this before -- or some version of it -- but here's how I think the book should have been concluded.
I would either have gone for no explanation, or have Jesus suddenly appear and take the dome away. He would have been cryptic as to why he did it. He would have said something like I'm sure you guys will be writing books about this for years; what was its purpose? And am I really Jesus?
Another possibility: the rest of the world is destroyed save for everyone in the dome. Again, Jesus arrives. He lifts the dome, and says, be fruitful and multiply.
If King was insistent on aliens, quite frankly, that ending triggers in my mind a different premise. Know all those videos on YouTube where people have one insect fight another? (And those are terrible videos, I don't know why people do that.) I would have the aliens have all kinds of Earth vs. contests, depositing their creatures into our world (tiny to them, huge to us). I would have literally had the alien kids filming these battles and then uploading them to their alien YouTube and generating viral videos. Eventually, the alien YouTube would ban those videos and they would stop. Humans would have no idea what's going on, but an epilogue would explain it to the reader.
I never read the book, but know the ending. So maybe some of this was hinted at in the book, I don't know. I kind of want to write that idea about the aliens and the YouTube-vs. videos as a short story. I hope that hasn't been done before (but it probably has).