Tokio Game Show is not far away... and the next Generation is around the Corner. But there are 2 reasons i think Japan Studio could reveal something big...

1. Everything from them announced is already released, so we know nothing what comes next...

2. They mentioned last year that they reveal something New this year, so what better place than TGS 2019 could be...

https://twinfinite.net/2018/12/sony-japan-studio-unannounced-game/

Toriyama-san, who recently worked on the aforementioned Astro Bot Rescue Mission, Déraciné, and Bloodborne, mentioned that in 2019 studio will be preparing to reveal an unannounced title in development, so he’s going to gear up for the launch of a new project.

I can't see anything big being announced on PS4 anymore

The Legend of Dragoon for PS5.

Ape escape with apes running around in different video games.

Demon's Souls 2

If i was Sony i would save any major announcements for the PS5 reveal or a E3 2020, yes i do believe they will have one considering its the year the PS5 comes out and it will need hype and marketing, whether its a proper conference like the previous ones or a Nintendo Direct sort of presentation.

A PSX in Dec 2019 maybe?

Sony could just "kite" out the rest, their lineup is still strong going into in the launch of the PS5...

However I doubt they do that, I bet theres still abit more in the tank.
And even after the PS5 releases, there will be cross gen games, that also go to the PS4.

Theres still tons of life left in it.

I don't. It's not like Sony only has one or two titles until the PS5. They have 3 SIE Worldwide studio games coming out, 4 first party titles, and 1 major third party exclusive (don't know if Sony own Death Stranding as an IP or not, but if they do that's 5 first party titles then). 

It's much more likely, honestly, that Sony only has one, or two cross-gen releases at most, especially if the Playstation 5 is backwards compatible with Playstation 4. I think Ghost of Tsushima will definitely be a cross-gen release on launch, and the only other title I think that will get an official Playstation 5 version is The Last of Us 2, which I don't even think would be a cross-gen release so much as a remaster 6 months or so after release. It's the last first party heavy hitter to release before the Playstation 5 comes out, so it won't be cross-gen at launch like Tsushima probably will be.

Third parties are much more likely to keep doing cross-gen releases well into the Playstation 5's early life. 

Don't See this happen this year, but on the other side you could be right