If I were Mazda I would not build a bigger rotary.
That is not going to buy them 20% better MPG.

If not building a 20% higher MPG turbo compound rotary I would focus
an a computer controlled manual transmission that shifted
super fast upon kick down to exactly the right gear
for the current speed such as this transmission from Mercedes.

They can also increase the power on the top end
with a trap door p-port and still meet all emission
regulations.

Paul Lamar

Hi Paul,
I agree with all of your ideas on efficiency increase, the turbo
compounding and especially the MB type transmission.
Why would a trapdoor type PP be any better than a throttle in the PP
induction tract that only opens at high rpm;
I cannot see the reason for such complication in the trapdoor design,
when it really is only a pop open throttle after all.
BR, Dave McC

The trap door, when closed, eliminates the induction track to exhaust
track gas flow path. This gas flow from exhaust to intake at low RPM causes
misfiring which puts out a dollop of unburned HC. That is why Mazda went to
side exhaust ports on the RX8 engine. It is all about emissions at low
RPM's low power. When the drap door is closed the engine operates EXCATLY
like a current RX8 engine.