Why not make a classic Arcade Mode where you have to beat the clock to the next checkpoint? And if the learning curve is too high, then that means you have a reason to make more levels, right? More levels would be awesome, and reaching a certain point could probably be a good way of unlocking a vehicle (or even unlocking a bonus vehicle by beating a bonus level after the final level, that has increased difficulty, another arcade staple). Adding time limits basically gives you a good idea of a slowly increasing learning curve that could transfer easily over to story mode quite well.
Many of the systems you mention are not currently implemented(meaning systems to handle the modes that are mentioned) in Nitronic Rush. The reason in why I mention this is that building a system requires time and a team to implement the system. The systems you have mentioned would require one or two programmers, and at least one designer to build it.
Sadly, most of the Nitronic Rush programmers either have other projects they are working on or are currently interning at a game studio (some even have jobs already). I myself (level designer) have a ton of projects that I have to do and don't have much time to do anything dealing with Nitronic. With that said, everyhting you have mentioned would be interesting to put into Nitronic, but it would just take too much time to do and we don't have enough people to actually implement large scale things, such as what you are describing, into the game.
Basically, we probably will not implement a new game mode, AI, or anything that would affect Nitronic Rush gameplay outside of creating a new level. That is where we are at right now when it comes to updating Nitronic Rush. But we may have more time to do something later in the year. Closer to summer.
It'd also be a good way of scaling up the city and making it seem even bigger than it is now. With each level you can create a slowly growing city block that grows higher and higher as you near the core, which could be even more massive than it is now, towering over everything right at the start of the game. Height could play a larger part towards the end in this way since you could zip through skyscrapers around the core district.
Having tall buildings in the last level would pull away from the grandeur of the Core building. Having anything remotely close to its size would pull focus from it. I could see ways of using buildings to building up to the core though. Or even having tall buidlings in the beginning of the level and small ones near the end, but that in itself would take playtesting and revamping to get right. Players probably would not notice it either due to the gradualness of the change(though that's just my opinion and I could be wrong).
Keep in mind that everything in Nitronic Rush is absolutely massive even though it may not look like it. The car is much smaller than the road it is on and the buildings are multiple road pieces in length, width and height.
Essentially, you could make a nice slope of difficulty on the main storyline without needing to have a hardcore mode. Or you could basically do the route that Cyber Speedway took and jack up every level to "advanced mode", and play through an advanced, complicated version of the Arcade mode's levels. That game had some JESUS FUCK hard levels towards the end if you played the advanced version. I remember Arcade games having these features and they were so replayable and addicting.
The hardcore mode was created because me and the rest of the team realized that most people cannot handle the rotation mechanic that the hardcore levels require the player to use(This was proven through playtesting at PAX, digipen and other locations).
The story mode was specifically created so anyone could play the game (6 year old was able to play the story mode at PAX
). I saw other young kids playing the game during the ProjectFUN workshops in the summer at DigiPen as well.
I should also mention that our tutorials would have to be much more comprehensive(feedback wise) before I would even consider putting the hardcore levels into the story mode. I admit that the tutorials are terrible and there's not much we can do about it now due to time and lack of man power.
With everything I've said, I would love to implement some of the things you are talking about, but there just isn't enough time to do so : (
Maybe something large scale will happen later on(closer to summer of this year), but not any time soon (at least for the next few months)
We're taking baby steps now. Bug fixes, levels, and possibly new vehicles. Maybe some server stuff for handling ghosts but that is it.