I think what can help you in designing a cosplay for a canon character, but one whom we don't really see (other than a glowing ball of light), is looking at the designs of other characters in the same series. In thinking back on other zelda characters, I think the choice of the lace around the neckline is a bit awkward. The lace doesn't seem to fit with the more stylized designs of zelda characters, and I think the satin finish is off, too. I think Navi is an energetic and bouncy character, and for that reason I think that collar thing really ages the costume. I'd either remove it or simplify it.
I'd suggest making the sleeves short for the same reason that Navi is a playful character. If you look at a lot of the Kokiri costumes, many of them have short sleeves (except for Saria, and if you'll recall she turns out to be the sage of the forest so it makes sense for her character to be more mature than the other kokiri). I think shorter sleeves would make the costume feel a bit more carefree.
I wonder if this could help you with addressing the "Navi-ness" of the costume. Also consider that Navi is from OoT and not some of the grittier/more detailed zeldas we've seen like Twilight Princess. OoT's costume styles have bright colours, and bold, flat, patterns. I'd keep the costume as playful as possible!