Sissoko is a consistent player (on a good way) but he plays on his highest level, I don't think he'll improve. Which is not a bad thing really, he's really good.
Mainly decision making. His on the ball skills aren't that bad really, has a good dribble (his strength helps him a lot). Problem is he often decided to dribble infront of the defence and then hastily play a shortpass inaccurately.