Ok, I have read this and all the following comments. As a strong Harry Potter fan, I feel obligated to explain this.
Firstly, when Harry dropped the resurrection stone in the forest, it was most definitely intentionally. He did this because he knew that the 'people' who were brought back to life were not real. Also, he didn't want to be the master of death, he just wanted a peaceful life. Some people have said that dropping an odd looking stone on the floor is hardly the best way to hide it, but no one other than Dumbledore and Harry knew it was in the forest. Even if someone did find it, it is likely they wouldn't know what it was.
Secondly, Harry broke the elder wand as he did not want to have it and be the master of death. Also, every wizard who has had it has been killed by another to try and get it. Harry did not want this as he clearly says that he has had enough trouble with Voldemort, and just wants to live a calm and peaceful life.
Next, some people are saying J.K Rowling has done this to create a sequel, even where maybe Voldemort comes back and makes more horcruxes. This is not true. The resurrection stone has shown that it can't bring back any wizard, just a virtual image of them. The only reason Voldemort came back the first time was because he had his horcruxes, so he never really died. However, all his horcruxes have been destroyed, so he is dead, and can never come back. J.K. Rowling may write a sequel, but I don't think it will have anything to do with the resurrection stone. People are just reading into this way too much.
Another thing people are asking about if why Harry has not destroyed the invisibility cloak. This is because the cloak was not created by the third brother to cheat death, it was instead created to be able to live a long and happy life, and to only see death again when he reached a ripe old age. Harry kept this as well because it was his father's, so probably has a high sentimental value.
People have also said that Harry becoming an auror would lead to him not leading a calm and peaceful life. Firstly however, in no point does J.K. Rowling say in the books that Harry eventually became an auror. And secondly, you say he must lose at least one of his battles, yet he managed to destroy Voldemort when he was only 17! I know it was all to do with the wands and such, but in the books Harry has shown that he has very high magical skills. First, he helped Ron knock a troll unconscious after only knowing he was a wizard for a couple of months. Then, he killed a basilisk in his second year. He won the tri-wizard tournament (and defeated Voldemort in a battle). He taught people his age and older how to defend themselves against dark magic. And he has one of the strongest patronuses, which he could successfully create in his third year! So, I think that's pretty spectacular.
So, basically, Harry drops the resurrection stone because he does not want to be the master of death. Simple as that. He does not want to be immortal, and he is not power crazy, like Voldemort.
I personally don't think that J.K. Rowling will write a sequel with high drama and Voldemort returning. If she did make Voldemort return, it would completely go against everything she has written about Harry Potter. I don't think she is likely to do that. However, she may write a sequel, but I think it would involve little drama and just sum up Harry, Ron, Hermione, and everyone else's lives after Voldemort.