I'm assuming you're talking mid lane.
In which case, I personally wouldn't pick either Talon, Kog'maw or Veigar. While Talon can be a great counterpick, you seem mostly focused on Mages and Talon requires a different playstyle. Getting stuck in Top lane because someone else decided to go mid (no AP!) will get you into trouble as well. He could be a good addition, but not what I'd focus on for now. I see Kog'maw more as an AD sitting at bottom. He has nothing in the way of escapes and I feel he's too fragile for a solo lane. Veigar is just a personal dislike. Objective reasoning would be that he's just a very late game champion and will have a rough time in this game which get decided in the first 20-30 minutes.
Leaves us with:
Vladimir, personally think he's a better top lane. He's a great pick and I'd advocate him for the same reason as Kennen: No mana which makes your jungler happy. Additionally, they'll both do top very well for a double WotA.
Swain: Can shut down some of the more common mids, but more importantly: is a very strong pick against Darius. For that reason alone he's worth considering.
Karthus: good pick. Just don't take him against enemy Soraka's.
Brand: Personally think you already got enough in his vein with Ahri and Cassi.