Firstly, sorry if this is a repeat... it seems everyone is being very hush hush about this tho!
For those of you who haven't found Okeanos Trading mod, this changes the price of imports and exports in the solo mode. It also allows you to sell your workers. This is a step in the right direction for a sandbox mode, weither it be user generated or from MC.
What we need is some top nerds working on how he's produced this... My vague understanding is that it is relatively simple XML that sets the trade prices... this XML is then encoded to the .patch file where Cities loads it from the PAK directory.
All we need to know is the structure of this file and the encoding algorithm and a simple GUI interface can be generated for making a custom version.
Lets unite and get this working! We deserve a sandbox mode, irrelevant of what MC think.