As mentioned in the feedback topic, we'd like to gather suggestions for future challenge themes. We have a few ideas in the pipeline, but would like to develop upcoming events particularly around your city-building interests. Most challenges thus far have been based on general themes. There's been quite a few, including skyscrapers, transport, rural and recreation scenes. In addition, we're considering introducing gameplay tasks, such as urban planning to solve a problem, or creating custom plugins to meet a set brief. So basically, if you've any new ideas, please also post these below. Even if they sound a bit elaborate or complex, we'd still love to hear them! The intention is to continue using the Gallery to submit image entries, along with the voting.
When posting an idea, it'd be helpful if you can provide: A brief summary:
What the challenge would involve / how it might work.
  Suggested rules:
If applicable, how the challenge could be kept fair. For some this might be self-explanatory.
  Target game:
Should the challenge be limited to a particular city-building game (e.g. SC4, C:S), or is it open to any?
  Recommended timeframe:
Suggest a reasonable submission period given the challenge requirements. The shorter events (i.e. 1-2 weeks) will be more frequently run, but the longer ones should be for more complex challenges.   Please post any ideas and they'll be added to the "Ideas Bank" below for consideration. Should an idea be selected, we may contact you for further information or assistance. You may submit as many as you can think of per post (feel free to post again with any new ideas). Thanks!
  Challenge Ideas Bank Themed Challenges
  Show of Traffic  |  Transportation  |  General rules  |  All games  |  ~2 weeks
  City Showcase  |  General theme  |  General rules  |  All games  |  ~1 month+
  Best Sense of Americana  |  Country themed  |  General rules  |  All games  |  ~1 week+
  City Replica Challenge  |  Recreation  |  General rules  |  All games  |  ~1 month
  Alpine Winter  |  Rural planning  |  Require snow, infrastructure built on hilly terrain  |  All games  |  ~2 weeks
  Sunny Destination  |  Aesthetics  |  Cannot use desert mod or palm trees  |  All games  |  ~2 weeks
  Vanilla Only Challenge  |  General theme  |  No custom content allowed  |  SimCity 4  |  ~3 weeks
  Space: The final frontier  |  Sci-fi  |  General rules  |  All games  |  ~2 weeks
  Ancient Civilizations  |  Country themed  |  General rules  |  All games  |  ~2 weeks
  Historic Development  |  Urban planning  |  Use an established city, with a historic background  |  SimCity 4  |  ~2 weeks
  Best Realistic "....."  |  Recreation  |  Aim to make as realistic as possible  |  All games  |  |  ~1 month
  Bridges  |  Transportation  |  Show one or more bridges crossing over something  |  All games  |  ~2 weeks
  Railroads  |  Transportation  |  Show a train on railroad tracks  |  All games  |  ~2 weeks
  Complex Design  |  Urban  |  Design an urban complex area (e.g. university campus)  |  All games  |  ~1 month
  Nature Scene  |  Rural  |  Only allowed limited use of man-made structures  |  All games  |  ~2 weeks
  Night Shots  |  General theme  |  Image must be taken at night  |  All games (with night mode)  |  ~2 weeks
  City Tile View  |  General theme  |  City tile must be shown in full  |  All games  |  ~2 weeks
  Waterside  |  General theme  |  Image must be dominated by water features  |  All games  |  ~2 weeks
  Photoshop Challenge  |  General theme  |  Image must be edited using a form of software manipulation  |  All games  |  ~2 weeks
  Non-Photoshop Challenge  |  General theme  |  Image cannot be edited using software manipulation tools  |  All games  |  ~2 weeks
  Animation Challenge  |  General theme  |  Image must be animated in some way (e.g. GIF)  |  All games  |  ~2 weeks
  CoriStar Challenge  [1]  [2]  |  General theme  |  Represent ST in a city's layout  |  All games  |  ~1 month
  Essay Challenge  |  Instructional  |  Create a written guide to answer the top 5 newbie questions about SC4  |  SimCity 4  |  ~2 weeks+
  How to do it?  |  Instructional  |  Explain the steps taken to create an image, so someone else could recreate it  |  All games  |  ~1 month
  Best Toll Road / Turnpike  |  Transportation  |  General Rules  |  All Games  |  ~1week+
  Simtropolis as an Acronym  |  Other  |  Create an acronym using the letters of Simtropolis  |  Game n/a  |  ~1 week
  Pinned Thread Overhaul  |  Instructional  |  Create overview guides to consolidate info in pinned threads  |  SimCity 4  |  ~2 weeks+
  GIFted Goodness Drive-In  |  Recreation  |  Create a city scene featuring a drive-in theatre, using an animated portion  |  All games  |  ~2 weeks   Custom Content Related
  Content Creation Challenge  |  BAT / Lot / Map / Asset  |  Require multiple images  |  All games  |  ~1 week to 1 month
  Facelift  |  Lotting  |  Enhance any default game lot, without changing the dimensions  |  SimCity 4  |  ~2 weeks
  Single Dependency  |  Lotting  |  Can only use one selected common prop / texture pack  |  SimCity 4  |  ~1 week to 2 weeks
  Modular Complexes  |  Lotting  |  Create an integrated set of lots  |  SimCity 4  |  ~2 weeks
  Themed Lotting Challenge  |  Lotting  |  Create a lot to a specified theme (e.g. parks, transport, RCI)  |  SimCity 4  |  ~2 weeks
  NAM Challenge  |  Content usage  |  Use the NAM for a specific purpose, such as to showcase or solve a set brief  |  SimCity 4  |  ~1 month
  Park & Path Set  |  Lotting  |  Create a set of parks and paths which can be used  |  SimCity 4  |  ~1 month
  Roundabout Filler  |  Lotting  |  Create one for both roundabout sizes, each with the same "style" per entry  |  SimCity 4  |  ~1 month
  Classic Model, New Lot  |  Lotting  |  An older model that has withstood the test of time should see a new Lot  |  SimCity 4  |  ~2 weeks
  Retaining Walls  |  Lotting  |  Create a new type of retaining walls  |  SimCity 4  |  ~2 weeks
  Transit Buildings  |  BAT / Modelling  |  Create a transit building model (e.g. stations: train, subway, bus)  |  All games  |  ~1 month
  Monuments & Statues  |  BAT / Modelling  |  Create a monument or statue model  |  All games  |  ~1 month
  Diagonals & FAR  |  BAT / Modelling  |  Create a form of diagonal building  |  All games  |  ~1 month