If you ask me, a much more interesting rival for Takeshi would have little or nothing to do with pokémon at all. Like others have said, he isn't especially the competitive type when it comes to that, and his career path is always treated vaguely even in the few instances that it has been addressed at all. His true passion of course, as has been made painfully clear over the years, is women.
Taking poison jabs to the gut was a pretty amusing take on the ancient running gag, but since it's a rival we're talking about, why not throw in some guy to compete for some of those young ladies instead (handsome pretty boy, hardass biker, you name the stereotype). If it came to that, rather than being put down by another girl, a little boy, or a pokémon, perhaps the flames of passion (and humor) would be kindled anew within our perpetually lovestruck fellow. It's not something I'd really expect to happen, but an interesting thought nonetheless - some of the most hilarious episodes ever have been centered on romance.