Races customization
The game with the best race selection I played so far was Master of Orion II.
The diffrent race mechanics which are being used in most games are either "same race for all" or "set races", where the player picks a premade race, with set units and buildings.
In MoO II the player had an option to "design" their race by picking traits that improved and weakened the race. This customization, as well as the wide variety of researches, buildings and unit customization, let players play the game the way they felt.
Elemental seems to have an ambition of wide researches, buildings and a very good unit customization. I personally think that race customization like MoO II and a wizard customization like MoM will let players play the game in any way they want, without feeling that something specific is better then the rest (assuming everything will be balanced, taking diffrent playstyles into account).