Please no Lex off the bat, would not mind seeing mxy showing yp to pester him and the family though!
I was actually thinking of this earlier today. Given how many traditional Superman villains and concepts have already been introduced through "Supergirl," the show might find it a challenge keeping things original and fresh in that framework.
Only partially kidding about Tempus. Given the love of and homages to the Reeve/Slater movies, I would not be surprised if a version of Nuclear Man shows up at some point.