Personally I would like
Mexican (obviously) league and Brazilian/Argentinian league. They would:
pave the way for an amazing Around the World Challenge.
add to the NACL experience and bring back the Copa Libertadores
bring some historic teams (Santos, River, Boca, Pumas, Chivas, etc.) to the game
change the monotony of the current game with a little latino leagues
Feeder clubs so I can develop my youngsters, help bring up a team I love or simply to receive loanees from bigger stature teams
Enhance the tactics
Better free kick experiences (specially in defense): where are players placed, who takes which free kick, how many players on the wall, etc.
Roles according to game status. i.e: when attacking allow certain defender to go up to midfield but let the other stay behind, or when defending have a lone striker upfront, etc.
creative freedom allowing me to set my players where I want (as in fifa) so I can maybe create a visible triangle among my players or maybe a square (like 1986 France)
Bring back the former regen system. Now you get 25 year old Morocans appearing at Monaco, or 19 year old Portuguese playing for Bordeaux. Before one would find hidden gems playing in the second division of Croatia or playing for a random small team in Argentina and it really made the experience much more enjoyable.
But most importantly INCREASE THE DIFFICULTY winning every big trophy year in and year out with a team that found itself ranked in the 6th division 10 years ago gets very boring and unrealistic. It would be really nice to have to fight for relegation with smaller teams for 2-3 years instead of having them playing the Champions League final 4 years after one takes them.