If I could pick some leagues it would be:
1) English Conference- Finally finish off England, with a proper FA Cup and relegation out of the football league.
2) German 3rd Liga- Add to the German leagues with a strong third tier, making the whole career mode more interesting
3) Brazilian Serie A- Awful it was lost in the first place
4) Ukrainian Premier- Probably the strongest European league without whole representation in the game, would be nice to see this change
5) Greek Premier- Another strong league not fully represented
6) Romanian Premier- Another strong league- would love to play as Steaua Bucharest or whatever they're now called
7) Dutch 2nd Division- Relegation in the Netherlands would make a more intriguing career mode again
Czech Premier- Bring it back, shouldn't have gone in the first place
9) Japanese J-League- If only they could get the league off Konami.
10) Portuguese 2nd Division- Relegation again would make the league more interesting.
One more thing that we needs more FIFA 16 Coins to buy these great player - Dinamo Zagreb, APOEL, Maccabi Haifa, BATE Borisov etc.