The idea of surpassing the other Shapeshifters is a good way to approach this. Obviously we aren't talking about Power, but of abilities that others can't even dream of doing.
Of course we don't have many Shapeshifters to observe, it's just Ella really and we can't be sure if she is that good with the trait, especially if she was supposed to be a failed half-half.
These are some of my ideas, you will notice that a lot of them are similar to the Creation trait:
We learned that the Dream World is the domain of certain traits. 1st, 4th, 5th, 7th, 10th and 12th. One interesting thing to note here, is that the "physical" Superhumans, tend to be more powerful than MC. For example, MC has weakness against energy based attacks (radiation and plasma stop his regeneration, electricity fucks him up, etc), Space powers can output a lot more force than he can (Alice can throw as powerful attacks at level 2, as MC can at level 3), Light is just too fast for him to contain, Truth has ranged physical attacks (like Eisheth's telekinesis which can crush him and limit his regeneration), Destruction just destroys his body on contact. Perhaps by leveraging the Immaterial, he can create intangible forms that are resistant or even immune to various physical attacks. And even if that's not possible, perhaps he can reach out and physically attack someone through the Dream World, overcoming resistances and damaging aspects of them that wouldn't be possible otherwise (similar to how Deus attacks can sever connections).
- So the first differentiation is that MC is capable of thriving in the Immaterial Planes as a Shapeshifter which may allow him to overcome his weaknesses against physical attacks, or bypass resistances that would normally counter him.
Memory is good at knowing.. pretty much everything. As long as MC can mimick something with his form, he should be able to be as good at using it, as the creature he took the form from. He can also use memories from other organisms to boost the output of the special abilities. We've already seen this happening, but sadly there aren't many Supernatural things with non-magical abilities we can observe. An example would be to combine the Eel form with the Leigong form in order to use the Leigong's crystals to channel electricity better. Or to find a monster with toxic blood and turn his own blood into a Supernatural poison that harms anyone who attacks him in melee.
- So the second differentiation is that MC can integrate any structure, no matter how exotic, in his form, and instictively know how to use it. Due to this he can mix and match ordinary and supernatural anatomy to boost the effects of new abilities, without the need to have the relevant Supernatural trait.
Right now MC feeds Memories to his tails to make them autonomous. I wonder how far he could take this idea. For example, feeding Memories to each strand of his hair. This isn't much, but consider what would happen if MC took a huge and abstract form. How would he command it? Every time we see MC turn into Giant, no matter the circumstances, he turns into a dumb pugilist and he is very slow. If on the other hand, his giant form was covered by sentient tentacles, then he could fight any enemy at any level. He could brawl the big monster, while his tentacles fight the smaller ones.
- So the third differentation is that MC can give commands to even his more abstract forms, offloading the mental load to sentient appendages that can do the fighting for him.
If MC has the potential to load memories into his constructs, and manage to combine this with persisted constructs disconnected from his body (like Ella taught him), then he can mimic the creation trait, creating sentient monsters with whatever memories he wants. He can also take it a step further and create things like virus, bacteria, parasites etc. He can invade other organisms and destroy them from within using poisons or even by delivering False Memories through these organisms, in a sense creating puppets of the infected.
- So the fourth differentiation is that he can combine Memory with the latest ability he learned from Ella, to create autonomous constructs, to various effects, from minions, to making puppets out of other enemies or attacking them from withing. All these constructs will have a certain autonomy, so MC will never have to mentally load himself controlling them.
Up till now, all these should technically be possible as they are already part of his powers. Perhaps the immaterial ideas are not fully possible, but at the very least there exists a possibility for alternative ways for MC to deal some damage.
But what about stealing Powers from other Supernatural organisms? I will assume that Xanthe is correct in that MC's powers will weaken if he transforms his lifeforce to mimick someone else's power. But there still exists a way for him to use it without compromising anything. By making constructs in advance and persisting them through the ability that Ella taught him.
So while it may not be a good idea to transform into a Fire Superhuman midfight, he can create a Fire-based Construct a week in advance, pouring all his power into it, and when he needs to fight, he will have a minion that is loaded with almost all his power to fight alongside him.
- So the fifth differentiation is the MC has the potential to create constructs that can use traits from other lines to assist him in combat or other tasks. The constructs will be complete, with the correct memories to use the powers efficiently, and with enough Power to match MC himself as there isn't any risk in pouring all his Power into them since he will have time to rest and regain it.