Personally, I think it's better with an open ending without the second part.

I agree... and I feel the same way about The Matrix ;).

I think Miles with the help of Ariadne made up a plan to bring Cobb back to reality or to influence his mind so Cobb won't feel guilty for Mal's death or something.

I really like this explanation. I think that it would have to mean that Cobb is still in a dream at the end, but at least a happy one in which he is reunited with his children and reconciled to his memories of Mal.