Investors may be enjoying the so-called Trump Rally since Election Day, but it could be coming at the expense of next year's gains, said David Lafferty, chief market strategist at Natixis Global Asset Management. Lafferty said he expects the global economy to gain some steam on improving U.S. growth and the "Trump Trifecta": infrastructure spending, tax cuts and a reduced regulatory burden. With better global growth, equity earnings should improve, in his opinion. While this could provide additional upside for stocks, Lafferty expects modest returns in 2017 as valuations -- and looming Fed rate hikes -- remain headwinds. He also said the post-election rally may have siphoned off some of next year's returns. As for fixed income, Lafferty said the back-up in Treasury yields makes high quality bonds look "less terrible."