My votes for a sequel. I actually enjoyed Superman Returns and feel that Brian Singer has a great love for the character. I really think he would do well with a sequel. Just make it more action oriented and bring in someone that poses a challenge to Superman. I quite liked Brandon Routh and I hope they keep him, no matter what direction they end up taking.
As far as the son thing goes, it could work. The villain finds out and makes Superman make a choice of who to save, the public or his son. Of course Superman can do both, but you know what I mean. I have a great respect for Brian Singer and feel he needs to have a second chance at Superman.