In addition to everything that has already been said:
I'd love to see improvements in the editor. For example, when we're editing player stats, the unusued space on the right of the screen could show player overall in different positions. Or picking up colors from the shirt, shorts, etc. that would be copied to custom colors to use again in other things. Simple things that would improve the game a lot.
New leagues would be great (personally, if I had to choose 1st division leagues to add, I'd choose some like Greece, Sweden, Norway, etc. or even CAF and CONCACAF Champions League teams like they did with AFC CL), but what I'd really like is to see the return of 2nd divisions (Italy and Spain) and the addition of a few new ones (Portugal, Russia, Brazil, Argentina, ...), or even 3rd divisions for England, France, etc. - this would be great for realism in Master League and BAL.
New leagues to edit - a second european league, an african league (instead of only one hidden team), etc... plus the ability of assigning as a 2nd division for some country which doesn't has one.
If fake/generic leagues bother people, just add an option to choose what leagues to use in Master League and BAL.
More national teams (even if generic) is a must. Currently, we have 80 or so national teams, and there are around 210 FIFA members. I'd like to have all FIFA teams in there, but if that isn't possible, at least, we could have some more african teams (we need 24 to make CAF Nations Cup, like Angola, Kenya, Gabon, Cape Verde, etc.) and asian teams (also 24 for AFC asian cup), even though having a full UEFA Nations League would also be nice (or at least some teams like Finland or Luxembourg). Also, some american teams would be nice, like Trinidad & Tobago and Canada, or even a few oceania teams to face New Zealand, like Tahiti, Papua New Guinea and Fiji or New Caledonia. The game would be much more interesting.
Stadium editor, or at least some more generic stadiums (like small stadiums variants with only 1 or 2 stands, or Sports park, Orione and Coliseo de los Deportes with covered stands, etc.) - and, please, more colors for seating! At least black! I'm tired of seeing black stadiums (Swansea, Hull City, etc...) with dark blue stands!
Ability to choose the region/country for a team. This would affect the transfers in ML/BAL, the language of the banners, the supporters at the stadium (caucasian, asian, african, etc.) and the atmosphere (chants, celebrations,...), etc.
Another cool thing would be a file type (like .ted or .ced) to import just a player. This would be nice if you want to import several face build or stats, for example, without having to copy them manually one by one, value by value, or importing a full team to mess up your editings. It would also allow the easier editing of those ML youths, classic teams, and many other things. It could be .ped (Player Edit Data), since .ted and .ced seem to mean Team Edit Data and Competition Edit Data, respectivelly.
Importing custom images for banners and flags. Importing chants like we could do in ps3 times.
In terms of gameplay, I'd like to see an improvement regarding the fouls and the referee. Seriously, playing against the CPU (I barely play online so I don't know what happens there) I almost don't get any penalties, and many times I see fouls that the referee doesn't seem to see. Also, the reaction of the players to certain circunstances need to improve. For instance, if a defender intercepts the ball to save a goal, they have to get it out of there quickly, and not control it like they are in the midfield. Passing sometimes is also weird: In attacking situations, sometimes, instead of properly receiving a pass, which would give them a big chance to score, they act weirdly, either standing there just looking and only start running much later or they just let the ball go to another player behind them.
This is what I remember for now. Probably I will remember more things later. If I do I'll add them here.