Favourite Elemental ideas ... so far

There have been some really good ideas floating about this forum and i'd like to get people's take on what their favourite ones are so far. In fact, there have been some really cool re-imaginings of older game concepts and some pretty well thought out new ideas too.

 Now i'm not a programmer so i've absolutely zero idea of whether any of these mechanics are even possible or not but as far as i'm concerned that's a good thing, as some of this would probably be 'out of the box' due to my ignorance of what's possible and what isn't. Personally there are three or four ideas that have stood out for me so far but i'm not going to grade them as i don't actually have a personal favourite (yet). And now without further ado, ta da da da da daaaaa

Growing cities; a very interesting and unique take on founding cities

Civilisation Coat of Arms; potential to add an extra layer of depth that could work on several levels

Specific v focused tech progression';  a re-imagining of the old that can offer the player some important choices to make

and two honourable metions; 'doctrines' and 'on finding shards'; combination of new takes on old ideas (doctrines) and introducing new ideas for a new concept (shards)

(I also really enjoyed reading 'the mechanics of warfare: weapons')