There's a needle to thread there, since power replication without limits would be excessive and narratively cumbersome, and diluted versions of most powers wouldn't be useful. I think Zekethor is onto something with the idea of tools. Even if their memory ability lets them reproduce abilities it makes sense that a capacitor or focus would be necessary to handle or adapt to high levels of normally incompatable monster energies. So something like the Undine heart to produce water, Cole's heat core to control heat, or Leigong crystals as a way to deal with electricity.
Syla was capable of those high level Memory abilities because she was perfectly compatible with the trait (at least at first), since Mc is a 50/50 then i don't think he will be able to fight like that. How WW will implement the Evolution powers is one of the things i'm most interested about.
About using things to focus different traits i think it's possible, we had an example with Mc using Deryl's amber crystal, he was able to use his own power to activate it, so maybe your theory is right.
Something interesting Nico mentions in this update is that all the strongest people in the Hero, and even outside of it, don’t rely only on getting stronger through straightforward evolutions. In her case, she has the Hexenringe, but she also says the other captains have similar special tools or mechanisms that boost their power.
So what would the MC have?
I keep thinking his main “tool” might be the memory abilities, but those don’t seem particularly useful for direct combat… Outside of that, he has some body modifications like the monster heart, certain absorptions, items from the shopkeeper, etc. But none of these feel like a concrete, defining power source. Honestly, the impression is that all of this just translates into stat boosts.
Ella definitely had abnormal things in her body to have that kind of overwhelming power.
So what do you think he could develop in the future beyond that?
Personally i don't think she means just "power" (the numerical value that can be measured in universe), but instead ways of becoming more powerful, in her case each hexenrige adds another portal, and can also (it seems) be combined to make larger ones.
In Mc's case i think monster part absortion is a big deal, they all increase his power (value) and capabilities (make his forms better). In terms of memory however i think the main thing (at least for now) were the paths, that connected him directly to all twelve apostles, it seems that he only absorbed a small fraction of everything they can offer.
Eventually i believe the use of Evolution powers will be the great difference for him, something unique that no one could replicate.