Consider maybe they won't include licensed content, stadiums and all that crap. They never do anyway. What can they do to improve gameplay? Watch a real match for example, that would be a good start. And play more Football Manager.
1) Pace. It's all wrong in PES 2008. Variable pace throughout the match would be a bliss, although I don't know how they could implement it in a 10/15-min game. A good start would be having the AI players not pressing and running like crazy throughout the entire match, having (most) AI teams give the f*ck up already and let go of the match when they're losing 5-0 at 70 min, etc. I can also think of an "anxiety" stat, like a momentum thing, that will make a side play faster but have greatly reduced accuracy in their passing, especially inexperienced/crappy players. I have many ideas like this to try and bring the simulation closer to the pace of a real match. Basically, I think players should have more mentality stats, and incorporate some of the FM teachings. If a player can't handle pressure, then maybe his accuracy and speed can be dropped a bit for important matches or tense moments of the match. Teams should also have stats as a whole, and this could be reflected in the in-game animations. For example, a lot of protesting and reprimanding teammates instead of clapping when a pass is not good. A general teamwork stat would be welcomed. When buying/selling a great number of players, at least some teams should take their time to adapt and improve their accuracy/teamwork. Runs and overlaps could also be affected by players not understanding each other very well.
Confidence or momentum is another thing I'd like to see implemented.
As you can see, there are a LOT of things that can be done to the game.
2) Massive AI improvement. This will bring about a lot of advantages, namely no more cheating to make the game harder, actually being a difficult game, and being able to intelligently implement things in point 1.
Basically that's it. Both points I wrote here are vital, and I don't think one can be implemented without the other. However, PES as it is now would be a good game with just point 2.