You get the option to build Colossus and Titans, you can send marauders to attack other empires. New options to start on a gaia world or a tomb world.
But I think the most interesting change is Unity Ambitions, so you can spend unity points not only on traditions but also on very powerful "edicts" which can even turn the tide of a war!

Ecumenopolis is the most important thing in this addon. Mega bonus to pop growth and all jobs production! Very interesting choice if you want to play tall.
The second thing is new megastructures with cool capabilities like insane minerals income or boosting unity, naval capacity (sic!), diplomatic weight or even additional envoys! You get a lot of fun with this DLC!