Ah I see! One moment...
So I swiped some notes from my host, because I remember him talking about this exact topic sometime last year, and honestly they apply to this situation pretty fucking well.
To start things off, implementing a crossover, no matter how you do it, is gonna involve work. Unfortunately, I'm knee-deep in my own shit so I'm going to pilfer this off on someone else, but in short, there are a few possible ways you can go about it:
1) Take a setting, and make it the Primary Verse. The idea here is you use someone else's world-hopping work and, well, hop off it. You'd want to use a setting that already handles world-hopping, such as Kingdom Hearts (shut.), Disney Mirrorverse (bleh), Magic: The Gathering, heck even Dresden Files if you're brave enough. Then the idea goes to adapt and essentially convert everyone's abilities to work within your source material. This has the advantage of having known and familiar mechanics, but it may not be what you're looking for.
2) as above but with your own setting. This lets you fine-tune everything to your specific calibre, but it's a LOT of work. At the end of it, you have enough of an idea how to deal with edge cases that you can start thinking of powers in terms of other powers. I imagine most LitRPG writers do this already with their Isekais as part of their Game Mechanics, but then don't bother crossing anything over cause they're not doing a crossover.
3) Don't have a "main world" at all, just evaluate each world's constituents with respect to each other. This is... probably where the most subjective bias comes in without MASSIVE rigor (the scientific kind, please). So you know all those nerds who argue on the internet on forums such as NarutoForums (if OBD still exists there), SpaceBattles, Sufficient Velocity, VSBattles, so on and so forth? Yeah they literally do this for you as part of trying to argue with each other. Main problem is either trusting forum warriors, or running lots of calculations yourself, oh and esoteric power systems are still bonk, like how would Chakra interact with Haki? (Most VS Forums uses
https://tvtropes.org/pmwiki/pmwiki.php/Main/MagicAIsMagicA across every fiction for this exact reason, and you probably should use it yourself... or not)
And then I think my host cheated here, but basically, it's the cop-out of "I'm not crossing over the different universes, I'm crossing over different AUs of the different universes, and that means I can change whatever, whereever, because AU!" Which is.... honestly I think this is just the second option with extra steps.
If I had to do this for NaNoWriMo? Yeah I'd probably model everything using something like GURPS or BESM (which is anime flavored GURPS basically, or... wait... FATE Core. That'll do it), and then boom now it's a LitRPG massive crossover esp with a narratively-focused system and..... hm... I don't need more ideas I have too little time as it is)
PS: 5) some other fancy way I didn't think of or read about, which would be super cool
