Call me boring, but I think I will get the most mileage out of orbital habitats. All those systems, that were hilariously useless, like a six planet system devoid of any resource and with no habitable body can at least produce science now.
Next up are the civics changes, they seem to offer more granularity, when it comes to designing a society. This and the ability to change the ethics of your empire seem to offer a much more organic experience, where old time allies may drift away from you, or enemies becoming friends over time.
Also, the ability to expel fungi.