There are to many to list and I'm assuming you're a newer follower because when I was more active I couldn't stop posting Chase/Leo head cannons. I'll list a few though! (Some NSFW)
If they ever stopped to think about their actions and how it affects other people, they could have made it as a couple without having to rewrite a bunch of stuff/remove the paranormal. I know a major point of them/their relationship is that they never stopped to talk and were bad for each other in that way, and 'simply' changing that is kinda massive, but it 's a facet of rushing into love young, especially in such a hopeless place like Echo with their miracle situation. But them being better at communication WOULD have helped a lot of things get better
If they didn't have their trauma pushing them into a sense of dominance/desire to be controlled, that ALSO would have fixed a lot of things
Leo/Chase break up in every universe at some point, even if they end up together again/'endgame'
Leo is more interested in bottoming than he seems but Chase is actually kinda slangin it
Chase never bothers to learn Spanish with Leo in any timeline
If Chase ever told Leo about Sam or Sydney in a universe where they were dating, Leo would believe him and try to help (either exercise Sam or coverup the murder)
Chase will intertwine his tail with Leo while he sleeps
Chase wore his ankle bracelet on his leg and that's why something always happens to them
Leo knows Chase is a peepee boy but cause he's a canine it doesn't register as weird
Leo LOVES the way Chase stinks because he's a raging homosexual and the masculine otter musk is doing its job
If they communicated more and Leo wasn't as jealous, he would LOVE to be in the cuck chair while Chase gets re-arranged because he would love seeing his boi happy (condom though bc only he can get Chase mpregged)
Leo can't get enough of Chase's otter squeaks
Chase thinks it's kinda hot when Leo gets a bit angry and overprotective (so long as it isn't controlling towards him)
The amount of garbage emo music they would listen to while cuddling belly-to-belly on bed after getting back together is absolutely staggering
In multiple universes they got back together after hooking up on Predatr at When We Were Young 2022 in Sin City
In most of those they broke up after a week once the MCR left their veins
I've never personally read Chase saying that Leo didn't like guns as "Leo is a liar about everything and was a gun loving republican the WHOLE time!" Instead I read it as, "Oh there is something WRONG going on with Leo that has caused him to go back on one of his core beliefs". With further information and retrospect, I think that was the intention with the scene. It explains Chase being SO shocked at seeing the gun, and feeds into facts we know and themes that can be extrapolated from the text as a whole as well.
IMO, the supernatural is a metaphor for trauma and Leo's is feeling responsible for Sydney's death, because he was supposed to be watching him. Thus we get Leo's desire to protect people and the supernatural feeding off of his desire to do so, which creates a feedback loop of negative emotions and mental agony.
Leo 'needs' the gun to protect people, but owning the gun makes him feel bad. As he feels bad, his negative emotions are amplified by a physical manifestation of trauma to make him feel worse, and makes his 'need' to protect people greater and greater, as his insecurities and fears are amplified over and over
This cycle, or 'circle' if you will, is one of the core themes of the game. The real narrator tells them to the reader moments after you hit start. Leo is hurt, and his hurt is damaging to himself and others. He isn't a monster that deserves no sympathy. He isn't JUST a toxic ex-cautionary 'red flag', he's just as hurt and in need of help as everyone else in Echo.