GTA 7. 6? Nope. Too soon.

FFXV
SE said its their most expensive game yet so I'd imagine its going to be packed with loads of content.

None. None of them will go past 50GB. Thats well down the road.

There's a Metal Gear Solid V joke here somewhere. I hope someone has already made it.

First one with 4K video

Nothing!

Developers seem to be avoiding using pre-rendered cutscenes recently. There's also a lot more room for compression.

Does anybody know if the PS4 could support more than 2 layer Blu-Ray discs?

None. A 50+ gb game would decimate HDDs, and devs would want to avoid that. There may be one or two near the end of the generation, but probably not.

Knack is 35GB, Killzone SF 39GB, NBA 2K14 42 GB, all launch games. 50+ GB games will happen. 1Tb console versions will come too.
Is a full install mandatory anyway? Why not run cutscenes from the blu-ray drive, or use it as a secondary source for loading data.

None.

Full installs are currently mandatory for all games to speed up loading times and such.

And as developers get better at optimizing and condensing their games, we'll see games get close to the 50gb mark but never cross it. Compression exists for a reason. No one would want to take 10-20% of their HDD space on one game.