If you have ever watched an episode of Grimm, then you know who David Giuntoli is. He was the Grimm. In fact, this is the role that makes me think he could be perfect as the next Swamp Thing.
In this role, Giuntoli played a police detective who learns about the supernatural world and finds himself a hunter of mythological creatures. His role is one of knowledge, power, strength, and genius. And those are all characters that we see with Alec Holland/Swamp Thing.
While David Giuntoli has been portraying a different type of character in the series A Million Little Things, it actually gave us an idea of how well-rounded the actor really is. He can be tormented and a hero. He is a father and a husband. And if he were to be given the opportunity to portray the Swamp Thing, we could easily see him bringing many of the same aspects of his characters from both shows to the DC Universe.
There is also a presence and grit to the actor that seems rather fitting of a character who loses their human face following an attack that leaves the character dead in a swamp, with their consciousness imbuing a humanoid creature who believes in the protection of all that is green. We can easily see Giuntoli stepping into this role with a level of genius and maturity that is expected of the character.
Although the actor is in his early 40s at this point, it still feels like he could take on this role for years to come. And since he would likely be the face of Alec in limited moments once he truly became Swamp Thing, his age might not matter that much in the long run.