A few suggestions...

Hi tiukengleng1994, welcome to the project and thanks for your support! I think most of your suggestions are very doable:

1) For commercial buildings to be connected together directly or by a linkway (of any kind) and for the player to be prompted whether to do so through a dialog box whenever it is feasible? (E.g. a (fictional) convention centre and shopping mall being joined together, see www.skyscrapercity.com/showthread.php?t=965532&page=2 (post #40) and www.skyscrapercity.com/showthread.php?t=965532&page=3 (post #41).)

I don't see any reason why this couldn't be an option, although I don't know if I'd want to be prompted for every possibility (could turn into message spam). My hometown of Minneapolis has a system of skyways that join the downtown buildings exactly like this. Would you see this as being just an aesthetic addition, or as adding something to the simulation?

2) Ability to have dual-gauge tracks so that e.g. standard-and metre-gauge trains can use a particular dual gauge track.

3) Have a seperate ground-level subway track option that is only compatible with underground and elevated subway lines (and not with standard ground level railway track like in SC2K).

4) Ability to mix-n-match railway vehicles' skins such that for e.g. high speed trains can have Shinkansen (bullet train), Pendolino, French TGV, German ICE, China HSR and Spanish AVE train skins all at the same time (or whichever way the player wants to configure the skins).

5) Have railway cars of differing lengths depending on how the player configs the railway line (best method through a prompt by right-clicking a particular route) or even depending on demand. Can also be applicable for HSR, subways, light rail, maglev and monorail (for the last 4 possibly only on the former point).

These would all be easy enough, although I think most of these would be something reserved for a more specialized rail-focused module, as most players might not want to deal with details like rail gauges. All things I'd want, though :)

6) Reintroduction of the zoning option for airports and seaports (like in SC2K and SC3K) with an option to build appropriate buildings and (for airports) runways like the latter game as well as organic growth of airports and seaports like in the former game.

Definitely this. I'd really like to allow finer-grained player control for large-scale installation like airports.

6) Ability to view a city at street level and allow the player's "avatar" to communicate with people living in a city. The former feature was available in City Life and Cities XL (although the former game's avatar was "unseen" by the player). See www.youtube.com/watch (featuring the rare Japan-only SC64 for the N64DD, in particular see 2:02 to 2:28 and 8:29 to 9:44).

A lot of people disliked this feature in CXL, but I think that was more because it seemed MC had wasted a lot of resources including it when more important things were neglected. Personally I think it's a good feature to have, although I wouldn't make it a core priority.

Great suggestions, keep 'em coming! ;)

I have a few suggestions also, to have 4 wealth levels.
 
I would love it if the wealthiest inhabitants of your city are vastly different from the poorest.
 
Here is a couple examples of poverty:

Here shows the contrast betwen the destitute and moderately wealthy:

Here is an example of obscenely decadant wealth:

Here is a wealthy suburb with the top two tiers of wealth:

This is a real life expample of the high density version of a mega mansion for a family of 5

I agree, there should be some metric of wealth for the inhabitants, and it would certainly be cool if it affected the appearance of the people walking around that neighborhood. Just as long as we can avoid the City Life/ CXL model where wealth level is predetermined.

If individual citizens or buildings can be upgraded in wealth levels in addition to buildings lowering in wealth levels then here is an idea based off of SimCity and Cities XL:
When first starting a region, the only people who can move in are $, $¢, and $$. Unfortunate poor people could eventually downgrade to ¢ under certain conditions. After a certain regional population or city education level is met, then the higher levels are unlocked slowly. Individuals can gain or lose one ¢ (1/2 $) every year based on various indices (satisfaction levels in Cities XL): starting income, education, employment, health with visitations to certain shops/leisure/service buildings and age as a factor (sims of working ages change the incomes/wealth levels of all sims not of working age in the household). An unemployed $ sim with low education will downgrade in wealth and health and can end up as a ¢. It is virtually impossible for a $$$$ (elite billionaire) to end up as ¢ (low life) unless he has been unemployed for at least 20 years, is uneducated, and frequents bars or casinos though it is possible for the child of a ¢ (so started out as ¢) to become a $$$$ billionaire over 8 years after starting work if he had the highest education possible before entering the workforce (presence of elementary and high schools), continued education after joining workforce (presence of high schools and colleges in the $ and $$ neighborhoods he moved to), stayed in good health (presence of hospitals and clinics near all residences, more healthy leisure/service/shop/venue locations than unhealthy ones (so more sports facilites/sports leisure buildings, grocery stores, organic restaurants, etc than bars and casinoes), and stayed employed at every step. Depending on education, health, and wealth levels (and possibly ordinances) sims will retire anywhere between the age of 40 and 120 (death). If an ordinance allows one to retire at 60 and forces sims to retire by 80 and the sim has low health or education, the sim will have 1 time drop in wealth by up to $ (one full wealth level).
 
 
Do you see how complex that is?
A more efficient way is to treat all workers living in a given building for over 10 years as the same and to have a flat (or semi flat) rate of population per workers (1:1 up to 10:1) have the same properties as the workers besides employment. Have residential buildings have satisfaction levels based on proximity to services and employment, have certain services have secondary bonus or negative effects, and have historic (or any building that is not replaced with another one) residential buildings shuffling workers to wealthier or poorer places unless that is impossible, in which case the buildings will upgrade or downgrade itself (downgrades were common in SimCity 4 Deluxe but mySim showed that the sims move to different buildings if education is high unless there were no suitible buildings in the city, then they just up and disappear as they move to another city). If you do not include any sort of individual citizen tracker (my sim or routing), then you can abstract that to buildings though having rediculous amounts of commuters should be rectified by having a ration of population to workers higher than 1:1.

@AzemOcram,
Nice pics of Monte Christo's City Life and the Antilla mansion here!
 
While on the whole i agree that there should be wealth levels included, i strongly suggest that wealth levels be automatically determined no matter what the location (like in SC4), by considering various factors e.g. overall desirability (of a section of city or particular 'hood) through factors such as transportation times, amenities, education, pollition levels, etc and the types of buildings should not be specific to classes only (like in City Life, where in the beginning you were unable to build hotels(!), and by having entertainment venues specifically tailored to a particular class only).  
 
As for your point on the complexity of the class system (esp. if it has to reflect real life as much as possible), i agree that it is indeed a very complicated system as evidenced by your last post here. The class system in the City Life game by comparison, was simpler, but overemphasised the education vs safety perspective (e.g. between fringe vs blue-collar and radical chics vs suits/white-collar) and class segregation. Your suggestion in the last post hence indicates that organic movement between classes is possible and can move beyond what is featured in City Life, without overemphasising on the stereotypes of class structure ranks.
 

City Life allowed citizens to upgrade in wealth from Centres of Continuing Education and without the socioeconomic class warfare, this can be used quite effectively. A combination of SC4, City Life, and Cities XL can make a very realistic education system where a building or worker's class can upgrade with high education and can downgrade with unemployment and low education.

What about allowing a function for indicating rail and subway lines as shown below, and allowing them to be exported as an image file (jpeg, png, etc)? Would look amazing and would lend an air of uniqueness to the game!
 

Cool pic! Definitely, I've always thought it would be really cool to have an automatic "Google Maps" style view of one's city.