My prediction is that gravel bikes will have wider forks and people will put MTB tires in them instead of 40mm "gravel" tires they do today. It will be faster, more comfortable and safer. Drop bars are great, they will stay. You don't need an MTB bar for your typical light off-road terrain.
Gravel bikes with wide forks and MTB tires already exist. That was my point with the salsa cutty example.
Classic gravel tires do not have very aggressive knobs, but there are some that do.
Gravel is odd as that can be hard packed dirt, small rocks, larger, up to mountain road (babyheads). For some of those conditions, the sidewall of a MTB is overkill and there is a stronger desire for a supplier, faster tire. Basically a cross country MTB tread.
So, there are already wide forked gravel bikes, and there are conditions where a 38 tire is plenty. To fit a 2 inch or 3 inch tire also requires trade offs for the down tube. Which is to say, it's arguably different bikes you want fir some Kansas gravel compared to northwest mountain passes.
As well, the upper limit will probably be at 2.5 inches.
Last, a lot of MTB tires are designed for folks that drive up to a trailhead and then ride down a trail. Those tires need to be solid, and just solid. So, I think it will be the variety of tires that change. More tires suitable for "light" cross-country MTB and ranging in between the spectrums.
