I would say, follow the IGH conspiracy. This doesn't necessarily mean that IGH, as such, is the villain. Honestly, the hints we've got so far suggest IGH is more neutral than that. However, this still leaves plenty of room for their unethical experiments to accidentally create someone who *is* the villain.
Yeah, IGH as a 'Power Broker,' working on creating meta-human enforcers for military or criminal elements to use and abuse, sounds interesting.
If they really want to tie it all together, the source of the 'power pills' used by Will (and later Trish) could have something to do with the chemical company whose products led to the empowerment of Jessica, and blinding of young Matt Murdock.
Even the Purple Man's parents might have been working for this organization, unbeknownst to them, or the prisoner experiments from Luke Cage might have tied into this company's illicit empowerment experiments.
Pretty much everybody but Danny has a built-in connection to this hypothetical 'Power Broker,' if they want to go with it, and Danny, through Rand Corp, can have some ties to it as well, or just tag along with his friends (since it all doesn't have to be about Danny, and he was already pretty heavily tied into the first Defenders season's arc, while Luke and Jessica were kind of tagging along).
Plus, the potential for empowered enforcers working *for* the company, including people with Cage-like skin, or, worse, Kilgrave-like persuasive abilities (although far lesser in scope, since that sort of power shouldn't be mass-producible!), could ramp the threat level far beyond 'Cage effortlessly slaps down some ninja.'
