Ideas I posted at facepunch:

I think people are looking over a really big part of what the Master Chief collection can hint towards. Think about it, every Halo Chief sided Campaign (1-4), every Halo multiplayer map (over 100 maps) all on one disc? Thats a lot of fucking data for one disc. We should be expecting a HUGE story in Halo 5. Imagine 35 different campaign levels? Maybe one perspective in chiefs, one in lockes, one in arbiters. The fact they squeezed 4 entire Halo games on one disc and can now make it a single game shows that Halo 5 could be the greatest Halo adventure, surpassing even the greatest expectations were having. COULD being a key word there, if 343 invests the time and effort, there is no excuse for what I just said not happening.
those last 4 games were built around last-gen consoles and technology. I don't believe they had the ability to push even close to 50gb of content for the last gen consoles. I could be wrong though. Halo 4 was pushing the storage with the amount of detail and visuals alone, (which is why MP was on a seperate disc) and the campaign was pretty short.