Well, last generation we had some pretty big games that used dual layers blu-ray discs (up to 50GB). Some good examples: Final Fantasy XIII - 38GB; God of War 3 - 35GB; Uncharted 3 - 40GB. Those were games that didn't reuse assets so much, like Read Dead Redemption, for example. So, with the current generation using 1080p assets, i think disc size for this kind of games will double. But wich will be the first blu-ray game that will show the message: "please insert disc 2"? Final Fantasy XV? Uncharted PS4? The Order: 1886? Metal Gear Solid V: The Phantom Pain? Quantum Break? Halo 5?

Definitely GTA 6. It will have mini games that would equal a full game...i think.

I don't think any of those. The new consoles have a giant amount of RAM, they don't need to stream raw textures from the discs anymore. They'll probably try to minimize required space as much as possible using compressed textures. Of course this will cause loading times but there are a number of ways to obfuscate these.

That was very called for, kudos.

I'm gonna say FF XV.

Yeah. Lots of FMV. Just see the trailers. I have a feeling FF XV will be the game and they will say: "See? We have the same amount of content as Final Fantasy VII, and because of that we use three blu-ray discs. And we needed 10 years of development time to achieve that."  

Well, Sony is working to get Archival Blu-Ray discs out in summer 2015. They are aiming to start with 300GB, then 500GB and finishing with 1TB http://www.vg247.com/2014/03/10/sony-unveils-next-generation-of-archival-300gb-1tb-blu-ray-discs/
''Over on Sony’s official site, the manufacturer confirmed that the first wave of Archival Disc Blu-rays will drop around summer 2015, and will offer 300GB of storage. Sony hopes to increase this limit to 500GB and then again to 1TB in future iterations of the medium. Blu-rays currently offer 50GB of storage.

Given their inclination to craft parts of a game as DLC even before the final product is finished, I am doubtful this will occur. They'll just put the extra content as DLC instead of disc 2 (joke) lol I doubt it will, but if it does, some major AAA game. Given the switch from AAA to indie (or lowered focus of the former), I think that mitigates the chances even further. I doubt MGSV would, but the next one with current gen systems in mind is likely to take up more space - though I doubt it will merit a second disc.

But, will the current blu-ray players read that kind of discs? Perhaps we will see it on Playstation 5 and Xbox Two.

I think few or no games had 25gb+ without useless CGI and stuff, so I'm gonna guess: it will take a while.

That will be interesting to see. They are described as Archival Blu-Ray discs, so why not?