An interesting theory that seems plausible:
It would explain why they're using roman numerals in the title for the first time.
The end of the of the trailer states that 'V' is awake. Now put that into context, I is Naked Snake AKA Big Boss, II and III are Solid Snake and Liquid Snake, and IV is Solidius. Leaving V to be yet another Big Boss clone. At least that's what I think, and both Liquid and Solidius had radically different voices than Big Boss and Solid Snake.
It would explain why they're using roman numerals in the title for the first time.