Penny is possibly the most underused character in the show, despite being part of the original ensemble. Whilst she was clearly intended simply as the straight man (er...woman) to react to the nerdy quirks of the other characters, not to mention eye-candy for the series, little has changed in all the time since. The writers treat the character with such dismissal that, seven seasons later, we still don't know her surname. Penny writing a play could easily be the premise of an episode: it's predictably awful (the play, not the episode) but Leonard pretends to like it up until the point where she unwittingly humiliates herself and finds out it's bad. The idea is never brought up again and nothing was accomplished. My suggestion is one episode where Penny announces her intention to write a play but is met with derision by her friends. She spends the rest of the episode looking for ideas, openly admitting that this is just a vanity-project so she can write a part for herself. However, rather than resolve it in the predictable way mentioned earlier, she never decides to stop pursuing the idea. Every so often for the rest of the season, she mentions the play or an episode somehow revolves around her attempts to complete it - e.g. trying to dispel writer's block by becoming a raging alcoholic or something. Towards the end of the season, Penny reveals that she's completed the play and sent it off to theatre producers, but refuses to let anyone read it so they can't tell her it's bad (cue sitcom-shenanigans where each character attempts to steal a copy of the play). The twist is that the play is actually good and a producer - who becomes a recurring character like Estelle from Friends - wants to put in on stage. The rest of the series run can revolve around Penny working to get the play on stage, dealing with snobby high-art types, critics and so on. The most obvious thing they can do with this is have her deal with the fact that she's not a particularly good actor and being sacked from the cast of her own play. It would give more dimension to the character, showing she does have talents, but wouldn't require changing her personality in order to be funny.
