The accomplishments would be reflected in the game world anyway, they have not done anything to make any of the guardians feel unique or accomplish anything. For example when TTK came out it didn't matter whether someone had killed Crota or not, the game world moved on as if they had. So any accomplishments your guardians COULD have done, whether they actually had or not, will be reflected as being completed in the story. So they really are just stat avatars. It's more important that the next game isn't held back by the previous game in any way.