You seem very obsessed with numbers! Less "pots", less CC's, less BHP's... the fact is that the car manufacturers will produce A) what people will buy and B) what they can get away with with regards to emissions.
Because less is always bad!!!

Who knows these 4 cylinder engines may be smooth as silk yet you've already written them off because they lack two "pots".
I have no doubt that they will take 2.0l 4 pots turbos to the next level. Because, at the moment, let's face it, the figures for 2.0l 4 cyl turbos isn't great. But it's not only the smoothness of the engine i will miss, it's the noise and it's the feeling i get from having an engine that not many people have. Every man and their dog has a 2 litre 4 pot engine and i just liked having something different and better.
I think PMKeates is right, this is an interesting time and could make huge changes to the way we live day to day.
To be honest, i don't think it is a particularly "interesting" time - quite the opposite in fact, it's boring. It's a boring time with everyone soon to be driving tiny engined 3/4cyl diesels because nobody can afford anything more. Although it would obviously never happen, but an interesting time would be saying to car manufacturers "Guys, make what you like.". Now THAT would be interesting.
Instead, now, you've got a load of bean counters saying *puts on Clarkson nerd voice* "Oh no Mr BMW, your car produces 126g/km, which is in contrevention to article 6 of the Environmental Bill 2012." Which is just..... boring?
The N20 isnt replacing the 330i engines, only the 2.5 is what I read anyway.
There is no 2.5 engine at the moment IIRC

. Do you mean x25i cars? If so, from what i've read there, nothing is concrete, but i think it's safe to say that there will only be around 2 6 cyl engines (if at all) in the F30 line up. I think there will be 1 3.0 6 cyl turbo (the 335i) with the M3 being a 3.2litre twin turbo.