Death-Stalker died by by partial phasing through a grave stone, Death-Stalker is powerful, but he doesn't seem very bright. I think people like Death-Stalker because he has a cool look, has some cool powers and had a cool death, but that's it. He seems more like a Ghost Rider villain, then a DD one. Again I can take him or leave him, but I think the writers would have work little to make him work in this context.
I actually think the Wildboys would be useful for more then a cameo, in real life street gangs have been willing to work for the mafia. The Kingpin and his organization may be at the top of the food chain in the criminal underworld, but that doesn't mean there are not groups below Kingpin's that wouldn't take orders from and do some low level dirty work. Let's face it, the poor youth of the Marvel version of Hell's Kitchen, wouldn't end up joining Kingpin's organization, they would end up in a gang by the Wildboys. Plus it makes more sense for Kingpin to having his drugs sold through a third party like the Wildboys, it makes him one more step removed from the process. The Wildboys are good starter villains, the guys Matt deals with at first before moving up higher on the criminal food chain.
Someone like Kingpin needs disposable people to do the dreg work of the underworld, dealing with the "clientele" he has no interest in personally dealing with. Let a street gang filled with foolish young men take the risks of committing crimes in public, if they get caught so what, plenty more where they came from and if one of really proves themselves and makes their bones on the street, then they might good to rise up and join Kingpin's organization. Its like in the Wire, the Greek and his organization sold drugs to the gangs in Baltimore and the gang leaders had teenagers selling drugs on the corners. The Greek was at the top of the food chain and was untouchable, because he did business with more open third parties.