This could be interesting, but also extremely annoying.
Imagine yourself not being that great at this game, and the rival being that guy who played 300 hours of this game and knows every nook and crany of it. And due to that, you make zero progress on the game. Maybe the progress you make ends up being negative, because the rival may kill you even earlier than the prvious kill.
I mean, there are ways around it, such as cycling the player who controls the rivals and all, so that he dosen't fight the same player twice, but I really hope there is a way to deactivate the human controlled aspect of the rival.
My (locked) thread about how difficulty should be a decision for the developers, not the gamers.