The S&P 500 (INDEXSP:.INX) has only just crossed over 2000 and Morgan Stanley analysts Adam Parker and Ellen Zentner are already looking to the next major milestone, arguing that this economic expansion still has enough gas to get us there. “An S&P 500 peak of near 3000 is possible should the US expansion prove to have five or more years left to it, based on 6% per annum EPS growth through that time frame and a 17x price-to-earnings ratio,” write Parker and Zentner in a September report. S&P 500 – Economy only now starting to expand: Morgan Stanley Even though the…