Err... SimCity 4... the core Maxis franchise over the past twenty years that spun off the Sims....

Network Addon Mod etc... hundreds of major and minor modification packs and addons... not to mention the tens of thousands of buildings that incorporate custom modifications beyond what the standard building creation tools are capable of.

The whole anxiety here on this forum about modifying Spore is unfounded; if you feel nervous about what modifications can provide for a game, just go buy SimCity 4 and visit simtropolis.com/modding. It's the franchise that basically got Will Wright his big break after all, if you like his games you should really give it a try.