First of all hello!

I'm usually a silent observer of this Forum, but the opportunity to give my own two cents about possible DLCs for this great game was to welcoming to skip!
As the others said: More Transportation Options (elevated Subways! Light Railways for Suburbs! Multi Track Stations! One-Track-Railways! Mono Rails! :O ) would definitely be welcomed because lets face it: Most of us are Transport Nerds, I'd think

But actually what I would also love to see (and from what I see, it would be unique in the City-Simulation-World) would be a little bit of political simulation. How about a City Council? We already have the ability to zone Districts, but what if these Districts would double as Constituencies for First-Past-the-Post-Voting (Certainly not the best but the easiest option to implement I would think?). So that regarding the social composition of a certain district a councilman from a certain party (Labour/Social Democrats - Liberal - Conservative perhaps?) would be elected to city council. City Council would then serve as a replacement for the current Happyness Indicator and some now free decisions would then be bound to what your councilmen want - Can you raise taxes when there is a conservative majority? Can you go for a law-and-order-policing-approach with the liberals? Would your Labour Party accept the slashing of free public transports to refill your mangled budget? We might even get so far as simulate an annual budget that would have to be approved by your virtual council. But well, since you are the one in charge of zoning the constituencies - why not redraw the boundaries so that the outcome matches your policies?

This would at least give the mayor back a bit of control here and there.
But to add a bit of difficulty: If the City really piles up debt and unhappyness there could be a Recall-Referendum giving you one year to turn things around before being ousted from Office.
Then of course there could be actual strikes or demonstrations on the streets regarding your policies, making you think twice if raising taxes in a poor suburb was the most sensitive decision to finance your new City Hall

Which of course would mean that yould would need to have the possibilty to fine-tune your expenses and incomes to each district.
Perhaps I'm getting carried away here now, but I would really love to see those features, without them becoming overly intricate of course! But I think that the quasi-political approach is something that most city-Simulations I know have lacked so far.