The following is an overview of the new features of the 1.3 update.
TL;DR - All mod origins have had complete reworks, with many civics being changed too.


PLAGUED SOCIETY
*18 new techs
*Over 10 new special projects
*Over 50 new events
*An archaeological site
*Special building available
*Special planetary decisions
*Special starting deposits/blockers
*A new relic

ARCOLOGY MASTERS
*Graphic issue fixed
*15 new events
*A special project
*Special starting deposits/blockers
*More balanced start

QUEST FOR THE SEEDERS
*Longer story
*Several special systems
*Over 55 new events
*4 special projects
*6 archaeological sites
*2 special anomalies
*A new relic

THE MASTER AI
*More balanced
*Special graphics for the AI
*Different effects

NEW CIVICS
*Machine Cult
*Nihilist Cult
*Crystalline Energy Grid
*War Economy
*Peacekeepers
(All of the new civics have special effects, and provide more than just bonuses)

BALANCE/FIXES
*Many civics now have negative aspects
*Several civics weakened
*10 civics now have special effects
*Many bugs/issues fixed
*Localisation edited

The list of features discussion page will be updated on the day of release.