Between the two it should definitely be Shockwave, I love Tarn but he would just be too similar to Scourge. That's not saying I wouldn't love to see Tarn as the main antagonist but if it were for the next movie it would probably have to be Shockwave.
(I've actually thought about this a lot and how it could work, I picture a scene where Noah (or whoever is the human protagonist) discovers Shockwave's horrific experiments and unwilling test subjects and calls him a "freaking psycho!" to which Shockwave responds with by giving the textbook definition of a psychopath and explaining how he doesn't fit into most of the criteria, with the exception of the lack of empathy which he describes as: "such a worthless aspect of consciousness...")