That's an intersting subject of conversation.
If Boeing continues making the 737, they'll name the next variants 737-1000, 1100, 1200, and so on...there's nothing wrong with that. That seems to be the logical next step. What will Boeing do after the 797? That's a good question. Many people have been asking themselves that question but that's not something to be concerned about because Boeing can still work on newer variants of previously designed models such as the 757 and 767. They can start thinking of new models such as the 757-400, 767-500/600/700, 777-400, and why not a 777 Freighter...and maybe the proposed 747-500 and 600 will be built depending on the market, all of that in addition to the two new airplanes left: the 787 and the 797. Maybe they'll announce a 747-700. So you see, Boeing engineers can come up with a whole lot of ideas for the next century, at least the next three or four decades !
The same applies to Airbus, what will they do after the A390? The A350 has not even been announced yet, although I think that if the A3XX becomes reality it will be called the A350, so they still have many new models to come out with before they design the A390. And they can come up with new variants of the A330 and A340 such as the A329, A331, A339 and A341. I'm sure they have on the drawing board many new models that people don't know about.