I think the whole point of the show (from the pilot on) is Leonard's pursuit of Penny. So he will get her in the end, but that doesn't mean they will not put him through Hell first. I can see them torturing Leonard with another guy who gets Penny's motor running, and even a proposal and marriage. Of course the marriage will fall through at the last second for some reason.
They have to be very careful though not to overdo it.. Penny's and Leonard's relationship might not be the central theme of the show but it's a sizeable part and an endless will they/won't they can only hurt the show.
It's ok for a short while if you can get something out of it.. case in point was recently Sheldon's "Please don't hurt my friend" which i consider one of the most heartfelt and emotional scenes in the entire show (especially because it comes from Sheldon) but if it's done for artifical drama it will quickly lose appeal.
I agree. The on-again off-again is getting tiresome. I would like to see them resolve the relationship and move on.