Yeah I agree, that's the only aspect I won't like though. Shadow Moses and The Big Shell were these intimidating structures which you had no way out from. In both MGS 4 and MGS 3, I found it to lose a bit of it's intensity because you weren't "trapped".
I'm hoping one of the bases in MGS 5 is so big that you get that feeling. The most exciting thing overall imo will be the transition from good guy to bad guy for Big Boss. Absolutely insane in the timeline.
MG - Snake saves Gray Fox from Outer Haven, as his commander, Big Boss, is revealed as a traitor.
MG 2 - Snake fights against Big Boss and Gray Fox, killing Big Boss in the fight.
MGS 1 - Snake fights against Liquid, who is revealed the be his brother. They are both revealed to be clones of Big Boss.
MGS 2 - Raiden is put through the Solid Snake Simulation, to recreate a soldier like Snake. The third clone of Big Boss is revealed to be Solidus.
MGS 3 - Big Boss as a rookie soldier. His boss defects against America (as part of her US assigned mission) and dies a traitor.
MGS 4 - Snake and Liquid battle once more (I don't like the whole Ocelot wasn't really Liquid thing so I pretend the last cut scene didnt happen).
Peace Walker & 'Acid?' - Missions by Big Boss after he has left America behind, now serving as a 'Mercenary without a border'.
MGS 5 - Big Boss goes from being the Mercenary without a border, with all the ideals of Big Boss, and turns into an enemy of America.