Going back to WWe will NEVER happen - he's hated by at least two of the top guys (Cena and Orton), hasn't come close to doing anything significant enough in TNA for WWe top brass to overlook that and his age is well against him.
RoH? I'd be highly surprised because I don't think his ring style would mesh and I doubt they'd be looking to pay his wage demands given that he has never proven any drawing power.
Japan? Ken's biggest attribute is his promo ability... not a factor in the Land of the Rising Sun.
Anderson is well down the TNA Totem Pole currently; so if he leaves, he leaves - it'll not hurt IW one little bit. But I really don't see any alternative for him than to re-sign with TNA unless he is sufficiently financially stable or has something else he can fall back on.