Tannin won't add body, it'll add astringency/bitterness and maybe some mouthfeel that adds to the complexity of the cider, but it will still be thin. The best time to add tannin is before aging (primary ferment is good too) so it can do it's thing and drop out with the sediment.
Sweetening alone should help bring back some of the apple flavor, but sweetening with concentrate will boost flavor even more and add some body.
Then if you plan on carbonating it that will add some to the cider too.
What was the original recipe, straight juice, or did you add any additional sugar/concentrate?
I used to have a six-pack, but now I have a keg.