The thing that will decide the longevity of the game is the first person gameplay (melee etc). Immersion comes from knowing that your actions, as an individual, have an impact on a battle and that the impact depends on your individual skill and playstyle. Moreover, people need to feel like they're improving and learning new things the more time they devote to the game, so there needs to be a high skill ceiling to keep them going. Everything else is a bonus: the scale, the graphics, the interactivity of environments etc. Without this you will lose players as quickly as you gain them.