Well, i don't think someone would develop a new CitiesXL. The amount of copies that were sold by Monte Cristo were below the expected and caused the company to go bankrupt. I dont know how much copies are sold by Focus Home, but that would probably be less than the original (just guessing here)
But I would like to see the following:
A more in depth simulation of the game; for example i have just a handful of shops for a city which houses around 900.000 citizens.
A more in depth traffic/transit simulation
More options for mass transit (like promised at first): subway, trams, buses, trains
A free design intersection tool for highways
Roads which allow the player to determine how many lanes it has
More different buildings
More modern landmarks, not just American/European. Think about Asia, Middle-East
A tool which allows to create your own buildings (by means of building blocks, like City Life 2008) which is user friendly
A working multiplayer with trading
And this is just a short list. Its probably way too complex to do all this, but that's what I want