Or are we going to see the Halo:MCC approach?

I was actually thinking about this a few days back and I wondered if devas would go for day one download of the overflow rather than go with 2 discs. I thought it seemed like a good approach especially if the overflow is 20GB or less. OTOH since games on PS4 at least get a full install the only disc swap will be on initial install and from then on only the play disc is needed. So 2 discs ain't so bad.

Or are we gong to see dual layer blue ray with 100GB capacity some time soon?

Maybe Metal Gear Solid 7 comes with 2 discs.

Maybe Final Fantasy XVII comes with 3 discs.

Imagine both games will install your console over 300GB in the future.....

Most likely the end of the generation. Only reason MCC is so big is because of the 4 full campaigns and 100+ multiplayer maps all upgraded for X1. I don't know if any one game will have that much content again.

MGS Collection. Uncharted collection might be one disk provided that they use real time cut scenes. Otherwise uncharted collection will be 3 disks.

Not very soon. The Halo collection is just an exception.

I think it will take a couple of years, maybe an FF game or something like that.

Dual layer BR disks have 50GB, you are talking about disks that have three layers.
The 66GB/100GB disks will become more popular in 2015 when they release the first BR players supporing 4K resolution.

It probably would happen with a Mass Effect collection.