We are due for an F-35 replacement

By due I mean its almost certainly already out there.

There have been 5 generation of jets since the 1940s. That is an average of one jet generation every 16.8 years.

5th Generation began in 2005. It's been 19.5 years now.

What to expect in a 6th generation aircraft:

Honestly, I think its going to be an everything jet. In the sense that it would be able to do everything any specialized jet in the 5th gen could do, along with having more automation and a possible no pilot mode where they can be flown without a pilot from another location or giving full autonomy to built in sensors.

In terms of elevation the jet will likely be limited to 50,000 ft just like the previous 2-3 generations of jets. Although I am hoping for an upgrade to 60,000 ft so that there is a feeling of scaling up.

It should also be faster than the current generation of jets, which is generally going at 1.6 Mach speed. That is around 1,200 miles per hour.

Hopefully this next generation we get something that can move at speeds of 1,500+ miles per hour.

The primary issue is that the human body can only handle so many Gs of gravitational force on the body and the faster a jet is moving the more Gs of force the human body is getting hit by.

The most convenient solution is to remove the pilot altogether and move on to autonomous jets or jets piloted outside the plane itself for 6th generation aircraft.

Conclusion:

There is a hardline limit on passenger craft speeds due to how many Gs of force the human body can comfortably handle.

It's not a tech issue, it's a biology issue.

Only a spaceship would at this point be able to provide faster travel for passengers in the aviation industry, as a spaceship leaves the atmosphere and lowers the amounts of Gs of force suffered by the passengers.

Robots will fight future wars at the limits of human biology combined with technology are reached.

The only way to make better soldiers now is to biologically enhance them.

We will see active biological enhancement of soldiers in the next generation followed by replacement with robots over the course of this century.