I think I can sum up the most important things to add in this post.
Career Mode:
An option for how long it is. I want to play all the games, but I don't want it to be always really long, for I don't have time to play it that much. Maybe anywhere from 5 to 100 years. Players age and retire. Young players can naturally improve (and a potential rating should be added, players with good potential such as Freddy Adu have a better chance of improving, when players get old or get bad injuries ratings go down. None of this training. It is unrealistic to take a random player and make him better at whatever you choose. A youth system please!
Realistic financial system. Players have contracts. When their contract expires, they have to be resigned. Of course if a player can choose to sign with a different team because of money or the ability to play on a higher level or displeasure with the team. I am confused by football's financial system in real life but their should be some way to deficit spend. All football teams are somewhat in debt, some do it too much and it can lead to ruin (such as my favorite club, Leeds). You should be able to lend and loan players. Computer teams should be able to be involved with each other in the transfer market. Companies pay to sponsor you, such as on your uniform (you also should be able to customize uniforms from year to year) or on billboards. If you're good, companies will want to sponsor you and pay lots of money. You should be able to build a stadium, set ticket, food, and merchandise prices.
Also, there should be coaching changes. Like Madden, there should be a preseason where you can choose opponents from around the world.
National competitions should be involved in the regular career mode. If you coach well, you get to manage the national team. Then, you can choose the roster of any player of that country. You can play cup qualifiers, the world cup, the Euro tournament, etc.
Please more leagues! They had some very poor leagues, like the Scandinavian ones, yet they left out some good ones, such as the Mexican, Aregentine, Greek, and Turkish! Some teams from these leagues are in the game but not you can't play with them in career mode.
For those of us that play MLS, for that league the financial system should work the way it does. In MLS there are limits to player wages, salary caps, and the youth is drafted from colleges.
Simulation needs to be WAY more realistic, teams like Real Madrid never make it far in Champions League.
You should be able to practice inside career mode. There needs to be captain select. More formations and strategies would be nice.
Gameplay:
In FIFA, the moves are terrible and don't work. They should change the system. In NBA Live, they have a freestyle control system. By pressing certain buttons you could do certain simple actions. The same should be for FIFA. For example, hold down shift and press a key and it does a stepover, or a chop, or a pullback, or any kicking the ball to the side with the outside of the foot, etc. However, the game MUST be realistic and the emphasis still has to been on passing.
Also, the passing and trapping should be more streamlined. It is difficult to effectively pass the ball through a defense and score. My goals are usually in the transition when my player gets past their midfield, sprints towards the goal with the defense backing up and giving him lots of space, then blasts it at the goal from 40 yards out. The only way to score is if you have lots of space, for it takes way to long to trap it and way to long to shoot it. if they make these actions just a little bit quicker, the gameplay will be way better and more realistic.
Other:
The menu needs to be better. You should be able to save replays, it really bugs me that you can't (or at least I don't know how). A creation center would be great, but the most important thing would be being able to edit players. There needs to be better cinematics. The 'feel' of a big game should be different from that of a normal game. If there is any way to increase the drama, it should be added. Maybe momentum should be added into the gameplay. If your team was down and now just scored 3 goals to come back, momentum should carry over. Or, like in WC2002, if you score an important goal, dramatic music should play. Anything that can be done to separate a regular game from an important one should be done.
You should be able to add mp3s and make it the menu music.
I think that's about it, hopefully I didn't leave out anything too important. If I did, I guess I'll just edit this post. Someone should forwards this to EA Sports. Do they look at this forum? They really should.
Edit: Champions League needs to be randomized.