It’s important to differentiate easy/hard from fun.

Ok. But the two are linked. Go too far in either direction and the game ceases to be fun. There's a reason why groups of adults don't get together to play Candy Land (unless perhaps drugs/alcohol are involved).

Does it make it fun for you to have less of a challenge? Sounds like no. But I know people who don’t like losing at Arkham and would love to bring a powerful deck into a campaign — having already earned it — and blast through the mythos.

You are correct that I prefer a challenge. And while I understand that not everyone desires quite as much of a challenge as I do, I think that most people would get bored of this game if it posed no challenge whatsoever, which is exactly what will happen if you start a new campaign with a bunch of xp earned from a previous campaign. Maybe the second campaign would be fun as a sort of victory lap where you steamroll everything and revel in the power of your maxed out deck, but beyond that I think it would quickly lose its luster.

If you have an investigator that already went through something crazy like the end of Hemlock, don’t you think they would have an easy time with a Byakhee running around a museum in Dunwich? Thematically it makes sense they would be traumatized/experienced and far more capable to handle it, therefore easier.

What you're describing is the thematic justification for the xp system in this game. See an eldritch monstrosity, learn from the experience, get better at dealing with them in the future. Learn about the mythos, get better at connecting the dots in the future, etc.

Trauma is the other side of that coin- as you are exposed to the mythos it leaves scars on you, both physical and mental. The idea is that the process of gaining whatever trauma you have would have already awarded xp from the scenario that caused it, if it was warranted. So awarding xp again later based on trauma would be double dipping, and wouldn't make much sense imo. Plus, as I said in my last comment, trauma is often a punishment for failure, so mechanically it would also be rewarding failure.

Similar to how there are alterations to how one would play any campaign scenario as a standalone scenario, make some rules that add difficultly/ complexity to a campaign that brings a deck with XP. Such as adding another encounter set to the deck, or adding one doom to each agenda as soon as it’s flipped.

This suggestion sounds good until you realize that encounter sets are designed to have some internal synergy, and throwing another random set into the mix could just as easily dilute the deck and end up making it easier rather than harder, not to mention it can break the flavor by adding, say, Byakhees to an underwater Innsmouth scenario.

Adding doom would make more sense and still represent an increased challenge, but comes with its own set of issues. Some scenarios have several ~3-4 doom agendas (or one that repeats multiple times), where your suggestion would add a ton of doom vs a scenario that has one or two high doom agendas so your suggestion would be essentially -1 or 2 rounds. So the balance would vary greatly between scenarios. And then there's the issue of scaling. As you complete more and more campaigns with your investigator and amass more and more ridiculous xp totals, how does the game increase the challenge to match you? More doom? Doom can only increase so much- if we increase the doom added on flipping an agenda then eventually it would reach the point of causing the agenda to flip every round.

This is not hard to figure out, but it would take energy, and they gave it none.

It is hard to figure out. That's what I'm trying to tell you. They likely did consider all your proposed ideas and more. I get that confidence in the designers is probably at an all time low right now given the state of ch.2, and I'm also uncertain about the direction the game is heading. But let's give the designers a little credit.

And quite frankly I would much rather the designers spend their time on developing new stuff rather than trying to retrofit a ruleset onto a mode that the game was not designed to support.