Before I give my feedback, I'd just like to say that this game is already awesome as it is! So, props for that.
What I'd like to see in future instalments is the following:
01) STATS/INFO SCREENS IN WORLD TOUR:
a) I'd love to be able to see injury indicators for CPU players, for example an 'player injured' icon added to world rankings next to the name of the injured player that when you hover over it or click on it shows you when that player got injured and how long is he/she expected to be out. Or, just add one single icon on the world rankings list that you can click on and see the list of all the players currently injured and dates when they got injured and when they're expected to return.
Also, it would be cool if you could see player's age in the world ranking screen and sort the ranking list by player age and position on the ATP/WTA ranking list.
b) Would love to be able to see more detailed stats for CPU players in the menus. For example, on their profile page, what is their highest singles/doubles ranking achieved and the date they achieved it on, list of tournaments won (sortable by date/tournament category...), what were their year-end rankings and so on.
And once you start the match, it would be cool to see stats screens like in real life: list of all the opponents that player beat on road to the current match, time spent on court, last year result at the current tournament, best result at the current tournament and so on. Also tournament stats would be cool: ranking lists showing ace leaders at the current tournament, serve speed leaders, serve percentage leaders, pts won on serve, most winners/errors...
02) PLAYER SKILLS/STYLES:
a) To further make players more realistic, it would be great to separately set up skills such as FH/BH flat shots/spin shots/slice shots, not only how strong, precise and consistent they are, but how often certain type of shot (flat, spin, slice) is used. This way it would be possible to create a more realistic version of a player like Steffi Graf who almost exclusively hit slice off of her BH side, or players like Nadal and Kuznetsova who use lots of top spin on their FH side, but play the BH side much flatter.
b) Similar to the above, I'd love to be able to set how often a player comes to the net (regardless of their playing style). For example, have a defensive player who also comes to the net to finish off the point or have an offensive player who is reluctant to come and play a volley.
03) GENERAL GAMEPLAY/MODDING OPTIONS:
a) The ability to start a season/career mode with one player and then switch to some other player in the same game would be cool. For example, I'd love to be able to play one season as Nadal, another with Federer and then another one with Del Potro while and keeping the history/records.
b) Ability to globally decrease/increase player's skills, especially the mental ones, depending on the surface would be neat. So that for example players like Sampras and Davenport would do well on grass, but not so well on clay. A modifier for playing in front of home crowd would be cool too, some players play better in front of the home crowd, some worse (for example, Mauresmo regularly choked in Roland Garros). Maybe even have the 'favourite tournament' parameter so that players like Sampras, Federer, Graf, Navratilova could do even better in Wimbledon, while Nadal, Kuerten, Henin, Sanchez Vicario could do better at Roland Garros.
c) Completely separate player's doubles and singles ranking in the .ini file, to be able to fine tune each individually.
d) Add a parameter like 'name_change: yes/no -> if yes, set new name and date of name change' would be cool in the .ini files, so that you don't have to create a separate entry/player for players who changed their names during their career. This would be good, because it would keep their tournament history and h2h history. Same thing could be done for the change of country. For example, Monica Seles played for Yugoslavia in the early nineties (prior to stabbing) and won 8 of her 9 grand slams playing for Yugoslavia, and then post-stabbing she won one more slam as a US citizen. So, it'd be cool if you could do the same in the game and keep that player's stats/history when change of country occurs.
e) For a more realistic impression, CPU controlled player should move towards the ball even if it 'knows' that the other player's shot is going to end up in the out or in the net. At the moment, if you hit a shot that is going out (regardless of how close it is) the CPU player doesn't move at all because it knows the shot will be a miss. This would be ok in cases when the miss is huge, but when your shot ends up just millimetres wide it looks silly to see the CPU controlled player not move at all. In real life, players would always move towards the ball (because they don't know for sure whether the shot is going in or out) unless the miss is really huge.
f) The crowd... A parameter like 'crowd favourite' would be cool in the .ini files, so that the crowd cheers stronger for some players than the others. For example, Roger Federer would have this parameter very high as he is highly popular all over the world. Stronger cheering for home players and underdog players would be cool as well.
g) Sliding on clay courts (and other courts too) would be cool to see, maybe even have a player skill for it. For example, Justine Henin was a great natural slider on clay courts, while the Williams sisters, Sharapova and some others weren't nearly as good. This should affect their in-game movement on clay (and/or other surfaces).
h) Hawk eye. This shouldn't be hard to implement (though I'm a programming n00b). A code like 'ball in/out within: 1 cm -> enable line judge error -> chance of line judge making an error = 0.10' seems like a good way to go about it.
i) The ability to customize the return position for each player individually would be cool. So that you can have Nadal standing meters behind the baseline waiting for the serve, and the Williams sisters standing inside the court when returning. Maybe make it adaptable to the opponent's serve skill (better server => more defensive return position; weaker server => more aggressive return position).
j) Human controlled reactions to balls that hit the net and go over need improvement. I'm playing with fast auto-positioning just because of this reason. It's like my player refuses to move when the CPU shot touches the net and goes over, it can be quite annoying. Not sure why it was set up like that, or is it me and I just suck? I'd actually rather play with average or even slow auto-positioning, but this is the only reason why I'm playing with fast auto-positioning and I still struggle with getting to these shots.
k) Ability to edit the full set of player skills in-game while in the World Tour mode would make life much easier, same thing goes for outfits and stroke animations etc. Also, ability to freely edit the skills would be neat (what I mean is to be able to change the power of the shot without its precision being affected).
l) Global skill change during World Tour. What I mean by this is, say you start playing in 1990, as you move towards 2000 and then towards 2010, the global skills (skills of all players) are modified (improved) so that as the time goes on, the game (tennis) evolves as well, it becomes faster, more powerful etc.