Prequel is a no-no to cliffhanger endings. Imagine if the next Half-Life game took place back on Xen detailing their attempt to escape from the Combine. Bad idea. And didn't we already get Ghost's backstory in that comic? Thought so.
If it is a prequel, I officially deduct 9,001 points from this game. Prequels are never fun when you already know what happens to everyone. Case in point: Star Wars. All those cool new Jedi are going to be dead by the third movie, so don't get too attached. If it's a sequel, then my hope is restored and I look forward to "Burn[ing] Moscow to the ground."