Ever since a NASA scientist announced Curiosity had made a discovery "for the history books" without saying-- yet-- what the discovery is, there has been speculation by scientists outside the mission and by nonscientists about what the discovery might be.
Most of the speculaation centers on results of a soil sample analysis Curiosity was conducting just before the announcement. Many scientists speculate the discovery might involve organic compounds in the soil, or chemical tracers related to organics. Organic compounds, of course, are the building blocks of life, so finding them on the surface of Mars would be a major find.
It could also put Viking results back in play. Viking possibly found Martian life in 1976, but the science community decided the results of Viking's experiments could have been produced by nonbiological procsses. Organics confirmed in Martian soil, however, might tip the judgment on Viking the other way.