All the main halo games had basic customization for Spartans. Halo 1 had basic colors, halo 2 had more colors and a elite character, halo 3 had a few armor pieces to switch between for elites and Spartans and halo 4 had an assortment of armor pieces. Should there only be one spartan across all multiplayer engines? Or the same customizations as they had when the first launched to retain the feel of the original version?
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In order to have your armor from lets say Halo 4 to carry over to Halo CE or one of the other three games that would require one massive engine change. They would have to code each piece of armor and armor combination to work in the games they aren't from. Then they would need to add the ability of changing our armor to Halo CE and Halo 2 which would be a massive change since it's not in there. Having the customization that's unique to the game it's from is a hell of a lot easier.