Justine Henin was the rival Serena should've had, but she walked away from the sport at a ridiculously young age. If she'd stuck around, I'd wager that Serena would have a few less grand slams. The paucity of talent in the women's game is just shocking. Federer is 34 about to turn 35, and despite still playing at a ridiculously high level, has not won a major since he was 30 (last Wimbledon 2012), because of the emergence of younger players like Djoko, Nadal and Murray. The fact Serena continues to dominate, while being a testament to her incredible fitness levels, is also a damning indictment of the rest of the field. You could easily see her dominating until she is 36 or 37.
Also, not that I'd personally want to watch it, but I just don't understand why women don't play 5 sets at the slams.