TF definitely wouldn't be a hard counter - he'd be a great ally, though. You could set up an entire team based on map mobility, ganking, and single-target pressure. i.e.:
Twisted Fate, Nocturne, Shen, Pantheon, Karthus. 5 Teleport. If you want, use Soraka instead of Karthus.
Regardless, you could group push top tower with Shen, Panth, and TF, then group ulti over to bottom tower where Karthus has already pushed out the wave. Nocturne snipes Baron ward and places one of his own. When everyone else gets to bottom turret, Nocturne ults, the whole team teleports over to Baron ward, and takes Baron.
Just the pure mobility of the team is hard to counter - the enemy team would have to be an extremely organized AoE team to counter it, and even that can have difficulties. See WCG finals game 1, CLG vs. SK Gaming. With only TF and Shen as global ult users on the EU side, they still took game 1 by just pushing and abusing their mobility.
Obviously you would need an organized team to pull this off. However, even played only moderately well, this would be extremely effective against any team that isn't incredibly organized and doesn't know exactly how to counter this - through pushing mid towers down and exploiting the weak team fights of the mobility team.
Just a thought. Karthus may not be the most desirable 5th, however. Part of it depends on how good of a jungle Nocturne is (cause if not, Shen can jungle). Part of it depends on whether or not the team needs an offtank.
