I've had this game and followed the development of Tennis Elbow for many years. I know that suggestions usually don't go anywhere, but I thought I'd put two obvious ones out there for the heck of it.
Just starting out online with Tennis Elbow, I've noticed two things that might really improve the community and overall fun.
Firstly, for a new player online, I get the impression that TE provides a kind of silent treatment. A lobby with chat and friends list would make the game a lot less intimidating for new players, and might lead to a larger base of online players. A forum might become obsolete compared to having a forum or chat integrated into the actual game.
Second, a kind of "TE Tennis Academy". An online practice mode would encourage cooperation, where "coaches" could teach newer players the ropes. You could work on just specific aspects of the game, learning to improve the serve, return, movement, volleying, various plays and strategy, etc....
I feel that this would lighten up the community and create a lot more cooperation, taking away from that feeling of silent elite competitiveness.