Well there's a lot to know just from the Steam trailer.

- There will be 2 new playable characters Marine and Pekora
- There will be multiple new boss fights. 
- It looks like there will be 2 new stages but I could be wrong that's just a guess.
- And I'm assuming there will be at least 1 more new end but that is also just a guess, it makes sense though that there would be. 

Outside of the trailer I don't know much about 1.3 mostly just stuff that is only relevant to speedrunning. But there are a few things worth noting.

- You wont have to pause to switch characters anymore.

- Marine and Pekora will be separate modes so you will not be able to switch between 4 different characters. It will only be Flare and Noel the same as before. 

- Cursed mode is gone, it technically counts as its own character so there would've been 5 characters but its been removed from the game now I guess. Not very surprising because it was something you only got from getting all of the achievements in the game and I think most people didn't know about it anyway.

That's everything I know about 1.3 aside from the speedrunning stuff. There could very well be a lot more that I don't know about, I'm sure the dev wants to keep at least some of it a surprise so we'll just have to wait and see.