There's a reason that most of the Punisher's best villains came from the Garth Ennis run on the character. Ennis focused on bad guys that we're on par with Frank Castle. You look at the Top 10 villains list that Marvel just released and you see characters like Agent Rawlings, Nicky Cavella, The Russian, Ma Gnucci, Barracuda and Jigsaw… all from the Ennis time on the series. This is important to note since we're probably weeks away from Marvel's The Punisher being released on Netflix.
The Netflix universe already has Kingpin / Wilson Fisk played brilliantly by Vincent D'Onofrio. And he's already met Frank (Jon Bernthal) in prison and helped him escape. The new series will also have Billy Russo aka Jigsaw played by Ben Barnes and Agent Russo played by Paul Schulze. Russo is one of the soldiers that Castle served with, but that doesn't mean that the series can't set up some others in the same way. The IMDB listing for The Punisher includes Raul Torres playing a character named Sicario. The word sicario means hitman in Spanish. There is a Hitman villain in the top ten whose real name is Burt Kenyon and he served with Castle in the military.
His military unit could be an easy way to set up characters like Hitman, Bushwacker and Barracuda if the writers want to or it could be just fun little Easter Eggs for fans.
This is the same way folks think that the "other" hero that was teased to appear in the series will be seen, especially if the other hero is Marc Spector aka Moon Knight.