For years and years the word was -- and JKR confirmed this herself -- that the last word of the last book (Deathly Hallows) was going to be 'scar'.
ES: Is the last word of Book 7 still 'scar'?
JKR: At the moment. I wonder if it will remain that way.
MA: Have you fiddled with it?
JKR: I haven't actually physically fiddled with it. There are definitely a couple of things that will need changing. They're not big deals but I always knew I would have to rewrite it.
MA: But it's definitely still on that track?
JKR: Oh definitely. Yeah, yeah.
MUGGLENET & THE LEAKY CAULDRON INTERVIEW WITH J.K. ROWLING - WITH EMERSON SPARTZ ( MUGGLENET ) AND MELISSA ANELLI ( THE LEAKY CAULDRON ) - 07.16.05
So this is a bit of a mixed message: Yes, the last word is going to be 'scar', except that I know I'm going to have to rewrite it, but it's definitely still on [the track of the last word of the last book being 'scar']. WUT?
JKR has never definitely ruled out writing sequels to the Potter books. My guess is that it's not on her plate right now, what with The Casual Vacancy and the publicity associated with that new novel. However, she has talked a lot about something called The Scottish Book, which is a code name for what would be her comprehensive Harry Potter encyclopedia (and what she went to court against The Harry Potter Lexicon for, to try and get an injunction against the Lexicon publishing their own Harry Potter encyclopedia. The court ruled against JKR and the Lexicon published what is basically a book version of their website.
I personally can't recall reading an explanation for the last two sentences in the epilogue. You can read some explanations about various characters and what she tried to convey about them in the epilogue in the transcript from her appearance at Carnegie Hall on 10.20.07 -- Teddy Lupin; Snape; Harry; Draco; to name a few.
The fact that the last word was supposed to be 'scar' for so many years, and then the last sentences were rearranged, suggests to me that she wanted to convey contentment and closure instead of focusing on a bit of flair that may not have fit the sentiment she was trying to get across. I don't see a "but then ... in a sequel to make me even more money ... muahahaha!" I think if she did do a sequel to the Potter series it would be because she felt compelled to tell a story, not make more money. She has so much money -- do you truly believe her motive for writing is financial? I can't quite go there myself.
¹Regarding Draco, this bit from the Carnegie Hall interview made me laugh: I think the very worst burden Harry could have put Malfoy under was this one, that Malfoy has to feel any kind of gratitude. So I tried to show that slightly in the epilogue when they look slightly at each other and there's a, "Hi. It's so embarrassing, you saved my life. No one will ever let me forget it."