Mind I ask, How would you ground Parallax?
Keep in mind grounded isn't a synonym for realistic. The buzzword for the Amazing Spider-Man series is "grounded" with the point being made that the two terms are different.
Personally, I'd have explored Parallax as a more Lovecraftian archetype, and like a Hyra, where if one head is cut off, two more grow in its place (ie Parallax as a singular entity would exist in separate places for my idea). Nothing the film presented showed why Parallax is the embodiment of fear.
A weakened Parallax, imprisoned in the Central Power Battery (due to an allegory Pax Lanternica, with many worlds feeling safe and free from real fear) uses the last of its power, built up and stored over millennia to break out of the Battery, a few Lanterns die in the battle, but Parallax is seemingly defeated by Sinestro and Abin Sur, how it actually imprints itself upon the Oan technology of their rings.
Abin Sur is charged with the honour of returning fallen Lanterns to their homeworlds (hence the spaceship) and then he is given leave to return to Ungara. Parallax then imprints itself up Abin's mind and then the battle from the 2011 film occurs with Abin dying and flying to Earth.
Hal inherits the ring and goes to Oa. Parallax begins to imprint himself upon Hal Jordan's mind, and sometimes Hal has flashes to his fathers crash, due to this Hal accidentally overcomes the yellow impurity (Parallax still exists within the rings of Hal and Sinestro, and therefore the whole corps). Hal begins to truly know fear (perhaps his temples grey?).
Sinestro still rallies the Corps together, but although together the Corps is invulnerable, this does not help with the fear of an individual Lantern. On Earth Parallax has infected Hector Hammond and slowly, the world, more wars are being fought globally, based on race, religion and diversity, things that stereotypically men are afraid of, the things they don't understand.
Hal eventually returns to Earth after coming and going, then going for a very long time, to find that it's on the brink of ruin, the US and any other major power are threatening to go to war (but both sides are actually to anxious to do anything), riots break out in cities worldwide the world economy has crashed because hundreds of millions are too afraid to do anything, even go out and work, the world is frozen in fear and society is deteriorating.
Hal, by this point has taken care of Hector and proven that people don't need to be afraid and, satisfied he's done his job, goes home and charges his ring. He speaks the oath, and the emerald light begins pouring out of the lantern, uncontrollably, but little of the energy is going to Hal, it's all fattening up Parallax, and crawling his way up from the depths of Coast City, having fattened up itself on the fear of the people of Earth confronts Hal. Hal, and the power in his ring will be the last to fall before Parallax challenges Oa itself.
Hal throws up his defences, but cracks in his shield begin to appear, and Parallax begins leaking in, streams of yellow gaining access to Hal through his eyes and nostrils, Hal sees his father's death again, and Parallax has its opening, it tears down Hal's defences and forces itself Hal's throat. Hal is gone, only Parallax now.
On Oa, many Lanterns fight with Parallax, while Hal battles him internally (Hal would now appear in Parallax's armour/visage). Hal eventually gains the upper haind in the battle for his psyche: he knows Parallax is the impurity, he knows that overcoming his fathers death allowed him to overcome the impurity before, and it will do so again, Hal finally learns to let go of his fathers death. Parallax is choked, raising Hal's ring to the CPB, and begins saying the oath, Parallax is flushed out of Hal's system (not entirely) and returned to the CPB (minus what exists in Sinestro's ring).
I know it might not be clear in what I've written, but rather than being something to be afraid of, I'd try and get it across as much as possible Parallax
is fear, the bad things caused are actions perpetrated by the hearts and minds of men.
I think in all the annuls of Comic-dom, GL has the easiest 'Reboot" of them all; just go with John Stewart and move forward.
Easy does not mean good. John Stewart is a boring ass character. Alan Scott would be better, not only is he a new character, but he also revamps the entire concept of Green Lantern. Also, just moving to Stewart and moving forward, isn't that the opposite point of a reboot? If you're moving forward, it implies it's in continuity with the Reynolds film.