The Passover Haggadah [quoting the Sifrei on Ki Tavo] assigns the identity of that Arami as Lavan ho'Arami
Referring to Soncino Humash P. 1118
Rashi follows the Traditional Rendering which is "..Laban.."
Ibn Ezra and S'forno identifies him as Yaakov Avinu
Rashbam as Avraham Avinu
So - believe it or not - the Rabbis do not concur about the identity of the missing Arami. :-)
At any rate - the underlying issue here AISI is in dealing with the disputing Hazal with regard to Parshanut in general.
Hazal HERE were talking ONLY al pi D'rash. Thus
1 As per Hazal - The Arami could have been someone other than Lavan al pi P'shat.
2. Ibn Ezra and Rashbam were free to fill in the p'shat which in Hazal omitted
3. Had Hazal indeed been talking on a P'shat level, we don't know if Ibn Ezra or Rashbam had the authority to contradict Hazal
[This assigns Rashi as saying D'rash and not P'shat]
Hazal were indeed talking "al pi P'shat".
1 Dorshin here refers to Midrash Halachah [Sifrei] - which is or can be P'shat as opposed to Midrash Aggadah
2. Ibn Ezra, S'forno, and Rashbam were free to dispute the P'shat because of shiv'im panim l'Torah, so Hazal's parshanut is not exclusive.
3. Even we [might] have the authority to offer our own version within certain parameters of good sense and good taste.
Pick your Parshnanut Preference
Thanks go to RDJM who assisted in this Post