With the current situation of the airline industry, my hypothesis would be an ultra-efficient, longer range jet with room for 125 or so *like* the A318. I don't believe the 717 can cut it, but this is only my opinion, and it is quite late here, so forgive my sleep-deprived soul.
Thinking about it, the oldest aircraft in production are the 757/767 lines (73,74,77 all are new/have new derivatives). Take into account the 764 and the prime candidate looks like the 75. But I feel this is far off into the future.