Well, the first two new SRGC had enough participents for me to keep on going. However, before I try to find another topic that is simple enough for very beginners - do you even want 'simple', or wouldn't you mind more complex stuff (multi-tile buildings, trains,...)?
There are many types of objects to use (stations & extensions, townhall, moving groundobject,...) or types of citybuilding (typical house of your nation, restaurant, factory, ...) or other stuff (Statues, Gardens, something from your favourite comic,...), it's just that I don't really know what people would be happy to join
Also, what about the time frame? Are there people who think a month is too short and would like more time?