Both Boeing and Airbus are spending a lot of time evaluating the next engine options. Last year there was an article that Airbus is more optimistic about CFM's open rotor designs while Boeing thinks the next generation geared turbofan models will win out. That is entirely based on leaks and no-one actually knows how true those assessments are.
The 737 Max was designed with the expectation that the 8 variant would be the sweet spot. Since that time it is clear that there is massive demand for up-gauging and the A321neo is dominating and there is significant demand for the Max10 variant despite it not being certified yet.
I would expect that both Boeing and Airbus are looking at that size (maybe slightly larger) for their next narrowbody with some flexibility for shrinks and/or stretches.
This is not a response to any existing planes. The A320/321 family is very old (50 years mid 2030) and it is expected that both Boeing and Airbus are going to be introducing new airframes to fit the new engine technology.