Zverev, Cilic and Delpo, if he qualifies somehow. Besides Novak, Delpo is Federer's toughest opponent on an indoor hard court, they faced each other eight times and split the victories. Delpo's back-to-back wins in the finals of Basel 2012 and 2013 and two RR victories at WTF were impressive, having in mind that Roger holds the number of titles record in each tournament and is an incredible indoor player.
I don't think that Nadal can beat an in-form Federer under these conditions (if Rog is not exhausted), especially after what happened this year.