0. New Deluxe skins, two new Warframes in the works (the one we already sorta knew about ie. Vlad/Revenant, and a Zephyr-esque 'frame codenamed Garuda). Also, Warframe is coming to the Switch, so that's also worthy of note.
1. As expected, Venus open world! - but nonetheless, full of really amazing stuff. The open-world area, a cold and frozen (but still livable, filled with Morrowind-esque mega-mushrooms) surface is called Orb Vallis, and there's also the underground colony of Corpus slaves (known as Solaris United) called Fortuna.
They even have their own theme song which is an absolute bop. It's like Dwarf Fortress almost!
New stuff you can do include DIY secondaries (Gunkits) and MOA companions a'la Cetus' Zaws, FREAKING
HOVERBOARDS, animal
hunting conservation, giant spider robots, and surface area which is apparently even bigger than Plains of Eidolon. Fun stuff!
2. Totally unexpected, the Railjack is basically FTL/Elite: Dangerous style of gameplay in space, where you and your co-op buddies get to steer and manage a spaceship (the titular Railjack), complete with going into space with the Archwing, flying right into the enemy ship, and destroying it via a combination of the person inside and the people on-board, complete with a daring escape afterwards as the thing explodes and crumbles to pieces. The fact it transitioned from the Fortuna/Orb Vallis demo pretty much seamlessly made it that much more amazing.
3. Last but not least, a teaser trailer for the next cinematic quest - The New War! Featuring the Sentients and post-Sacrifice Lotus, there's a chance we'll finally get to go to the Tau System ie. the Sentients' de facto home system they were sent to colonize ages ago...But I personally think it'll involve a full-on Sentient invasion of our System, and Railjack-powered space combat. But who knows!