I still think in the USA, and perhaps in other parts of the world, direct gas injection engines will remain popular.
However, outside of the USA, diesel is the way to go due to taxes, and outright power production (Tourqe) for day to day use of the vehicle. (Besides the slightly better fuel economy because of the additional energy contained in diesel v/s gas.)
That being said, the idea of a VATN setup would be very nice indeed.
Even the possibly simple idea of using a divided hot side with a valve that forces all the exhaust gas through one side of the housing, promoting high velocity at low RPM, while opening up under more exhaust manifold pressure, and allowing for high flow rates, to bump up top end power as well...
Personally, an engine of about 3.0L, with a static CR of 11:1, direct injection, and a T-70 turbo with a VATN setup would be amazing.
Off boost, the high CR would make it drive quite well, plenty of getting around town power, and very responsive.. and the engine could be designed to "switch" over to a almost diesel like operation where the fuel is injected only when the flame front is wanted, just prior to TDC when the spark would normally ignight the air/fuel mix.. (And if you tried this on a port fuel injected design, the high pressures would light off that mix way too soon, and in a out of control manner.. Detonation basicly..) But with direct injection, you could run the engine at idle, and off boost like a normal 4 stroke, and have the smoother idle, mid point power and quiet engine operation.. but under boost, and high boost conditions, say 30psi... you basicly run the "gas" engine like a diesel.. and only inject the fuel when you want to burn it.. and the hot compressed air would light off the incoming fuel, but if it did not, you could use the spark plug to assist as well...
It's a very interesting concept, and I'm supprised nobody is working on something like this, and making noise about it. (It solves many pollution issues, and NVH issues consumers have about diesel engines in this country.)
You would need a pretty long stroke engine design to improve on the amount of time available to burn fuel... So it would not be a high rpm engine, but it would be higher than most availble diesel engines...
Again, I'm just thinking out loud..
One more thing.. with the variable timing control, you could have idle valve overlap under "normal" gas operation very light, so few unburned HC's escape out with the exhaust, but when it's on full direct injection mode, under boost, the overlap could be insane! With both the intake and exhaust valves open, your forcing fresh air out the exhaust to some extent, and this would help to burn off any remaining HC's, could result in higher EGT's however depending on how much "Air" is added to the exhaust flow, but in theory would improve tailpipe readings to the point where perhaps a cat is not needed? Or smaller cats could be used?
The final thing is incoming pressurized air at high rpm, under high boost, at the end of the exhaust stroke should help to slow down the piston, and not just rely on the rod to do all the work.. Making long stroke engines possible with this type of valve and fuel control...
Again, just thinking out loud..