Some random thoughts about the career mode (those with a * should definitely be features imo):
- *Doubles partner affinity: Makes players more likely to pair up with someone they like more, perhaps also hard-coded with real life doubles pairings, especially if they're related, like the Bryan brothers for example.
- *Doubles specialty: Think of a way to buff the stats of mediocre singles players when they play doubles, especially if they're good at it in real life, so we don't get singles players like Maria Sharapova dominating the doubles tour.
I think these two things above would tremendously improve the experience of playing doubles in career mode, since it's a real mess right now and it shouldn't be, since you went through trouble of making it a feature.
- Player relationships: A more complex version of Doubles partner affinity, where players have a like/dislike rating towards other players, maybe even developing rivalries and giving a slight buff to stats when playing a rival (like when Ivanovic plays crazy good against Jankovic). It doesn't even have to be automatic. Give an option to periodically improve or reduce the relationship of an opponent you faced during that period, for example. Pretty cool for role playing.
- *Fed/Davis Cup player selection: Make an option to change player selection, especially in doubles, in addition to the already present option of picking players based on their ranking. It's pretty annoying when you lose because some nobodies get picked for doubles.
- Final set tie break in USO: Honestly, this is irrelevant to me, but it would reflect real life.
- Surface specialty: Make it like a quest to develop a specialty which gives a slight buff to player's stats on a particular surface.
- Serve buff based on height: Not sure if this is already implemented, but tall players should serve more powerfully than short players with the same serve stat.
- Speed buff based on height: Same idea as serve, just buffing shorter players.
- Dynamic weight: Make it possible to change it slightly during off season or something. More weight giving you a bit more power, while less weight giving you more speed.
- Fatigue from overplaying: Again, not sure if this is already in the game, but playing 3+ consecutive weeks should de-buff your stats temporarily, or make you lose stamina faster during matches.
- Confidence: Get a slight temporary buff from consecutive wins, titles or a Top 5 win and a slight de-buff from consecutive losses.