WARNING: The following contains spoilers for Venom #30 by Donny Cates, Luke Ross, Jesus Aburtov and VC Clayton Cowles, on sale now.
It's hard to imagine how a villain who conquered the world, brainwashed the Avengers, and forced the remaining heroes underground could become even more intimidating. But in Venom #30, Codex found a way to be even more terrifying by discovering interdimensional travel after doing all of that. This would have given him access to an infinite amount of alternate Earths that would have no way of preparing for Codex, who would undoubtedly launch a surprise attack against said worlds.
But while Codex may have been redeemed in the same issue, the potential implications of this are too great to consider, as well as how it may come into play now that Knull is coming to Earth.
In Venom #30, Codex was trying to find Eddie Brock -- an alternate version of his father -- he was interrupted by Otto Octavius. Otto, as it turns out, had learned from Virus' suit that he had come from an alternate Earth. More importantly, he realized that he could replicate this technology within a week's time. Even in his anger over being interrupted, Codex immediately grasped the greater implications of what this meant for himself. He ordered Otto to get to work on constructing this device. Thankfully, Codex was saved from Knull's influence by his family and the threat of him coming to other worlds was eliminated.
If Codex had not been redeemed, the potential damage Codex could have wreaked upon the multiverse as a whole is incalculable. For starters, he is a world conqueror, who managed to subjugate most of his world within the span of a few years. His powers managed to overwhelm the likes of the Avengers, not by killing them, but by turning them to his side. Those he conquers become weapons to use against his remaining enemies. And the only real chance to stop him would be to get through his army to attack Codex directly.
This leads to the next major problem in that plan. Codex controls all of his symbiotes through a hivemind. His connection to Knull hints at the greater powers he has inside. Codex comes from another universe, yet he has an innate connection to Knull, who is native to Earth-616. This suggests that Knull's influence reaches across dimensions. And if Codex has similar powers, then he wouldn't even need to enter these alternate Earths himself. He could just send the vanguard of his army and more symbiotes will be produced shortly after. He'd be the ultimate invasion force: a commander with an army absolutely willing to do whatever he wants with no limit on replenishing its numbers.
Thankfully for the multiverse, he was stopped before he could do any of this. But now the technology exists on his Earth. It was developed to get Eddie and Dylan Brock to their reality, just in time for Knull's invasion. But even though Codex is out of the game now, his powers still remain. If he wakes up sometime during the "King in Black" event, there is a good chance he may want revenge against his former manipulator. So it is possible that Codex may lead an army of his own symbiotes against Knull in an ironic turnaround.