He's been around since the Golden Age, appeared in multiple animated and even live-action incarnations (coming soon to Gotham!) and even had a long stretch as a sympathetic character in Starman. But I think he's a horror franchise waiting to happen, based on today's book: from 1995, Batman: Shadow of the Bat #39, "Solomon Grundy: One Night in Slaughter Swamp" Written by Alan Grant, pencils by Kevin Walker, inks by Curt Shoultz and Frank McLaughlin.
I was slightly surprised by the body count this issue. There's a brief recap of Cyrus Gold's origin--or rather, his murder: lured out to Slaughter Swamp by a blackmailer, then murdered and dumped into the bayou, where he would return as (usually) murderous zombie Solomon Grundy. Recently in DC continuity, Grundy had been reduced to petrified wood by Green Lantern Alan Scott and his daughter Jade, which admittedly does seem like a solution that would take care of most problems; but Grundy had made surprising returns before. (It's pre-Crisis, but I'm pretty sure Superman left him on another planet, and Grundy had been stuck on the moon at another point in his history.) Somewhat unsurprisingly, Grundy returns, screaming "What's my name?" as he vents his fury on a passing tour group, killing several, including a young boy's parents. The boy escapes with a park ranger, but they run into Grundy again later and help him remember his name; not that it makes him any more amenable.
Batman tracks Grundy down, albeit too late to save the ranger; who we learn in the conclusion was a descendant of the blackmailer that helped murder Cyrus Gold. Bats is at a double handicap here: the swamp is Grundy's home field, and Batman is unwilling to let even a murderous zombie die if he can help it. (I picked up this and Shadow of the Bat #0 from the dollar bin; and both made a strong point of Batman protecting all life.) The young boy, now orphaned, is at least saved; but I wondered if he swore to devote his life to vengeance against Solomon Grundy...
This could work as a movie (hopefully better than the similarly swamp-set pre-MCU Man-Thing) or short series: an environmentalist group protests to protect Slaughter Swamp; counter-protesting is a growing group of aggrieved friends and family members of Grundy's victims. Throw in some industrial types who want to drill for oil or drain the swamp to build a golf course or something; and maybe some criminals using the swamp to hide out or for smuggling; and season with a murderous albino swamp zombie. Some believe in Grundy, others don't: the industrialists may think it's a boogieman, or a smokescreen from the environmentalists, who may think Grundy is an avenging spirit of the swamp--and he's not. The families of Grundy's victims may just be waiting around the swamp with high-powered rifles and flamethrowers...I'd even play with the idea that you can kill Grundy, but he'll return with a different personality: if you're able to kill the Jason Voorhees-style Grundy, he might be back as a Hannibal Lecter-type later; and he might return in six months, or almost immediately. And don't kill a harmless Grundy for revenge, and have him return as a cannibal! Potential, although it'll never happen.