This may sound odd, but the best thing for FIFA is for PES to be decent again.
EA have rested on their laurels far too long and for me the series took a nose dive in the gameplay department from Fifa 12 onwards.
What EA need to do is massively improve their AI, and to stop the game feeling so claustrophobic in the midfield/final third of the pitch. The games just end up like basketball matches swinging from end to end, there is no real decent build up play because the game isn't designed to be played that way.
Slow the game down, tone down the contain even more, allow for matches to have a variety of playing styles, not just kick and rush with a mixture of lobbed through pass for a 1v1 more often than not.
They have everything else in place, but the gameplay is the big area FIFA fails for me.