abilities:
for traveling to different worlds. I believe that the world eaters would have a more powerful version of this. When they are in the space in-between worlds, that means that all the laws of physics don't apply. As you described, he would be an outsider looking in; this would mean that before the world eater enters a world, this being would have the option to travel to any when and any where. Of course, he can only travel to any when, when he is outside of the world, he would still be able to travel anywhere. I believe he would act as a singularity and that other world eater and time travelers wouldn't be able to travel through time.
Since the world eater's main powers are space and void, this being would easily be able to hide his entire being and be able to travel anywhere. This being would be able to turn himself/herself intangible and go through anything solid. This being would be able to travel through the void to shorten the distance in whatever plane of existence he is in, as well as to travel to other planes of existence. The reason why he/she would be able to travel like this is that every plane of existence would be connected or can connect to the void.
Since this being has a high control in the space element, it would make sense for him/her to create a pocket dimension to store his/her stuff.
As the saying goes, 'you stare into the void, the void stares back.' from this, he/she would be able to tell who is using a divination spell on them, or in other words, spying on them.
as for the system, you could base it on a black hole or something similar. he just needs to keep on consuming whatever to keep on getting powerful; matter would be converter into energy, even light would be converted.
What do you think so far? I still have a few more thoughts on the matter if you want to read them.
Time is something i dont want to mess with. Maybe each world will have different time dilations but going to the past is impossible in any way shape or form. At most you can slow down time, travel to the future (enter stasis and reappear when the time is right), or accelerate ageing.
When he first enters the world he has a near omniscient view of the world and chooses his thief friend through that. Afterwards he restricts himself with the worlds laws and becomes a mortal. He cant initially use his powers unrestrictedly because his mortal body would break down (his space rifts are hardly smooth tunnels, and pass through chaotic void) . Pretty much he starts off very weak (unless he gives up his mortal body and the safety of the world) . Of course this doesnt restrict his mental passives. He can see the world unlike any other creature, directly observing the rules and reality of the world (though he doesnt necessarily understand what he sees). All the space spells and void abilities are things he can do as a world eater or/and will eventually learn to do with the worlds magic/power system.
I dont intend for anyone to travel between worlds most of the time. The space between worlds is pretty boring and empty, and there is no reason to travel to another world until he finishes his business in the first world (not accounting for minor realms inside the world).
the anti-divination idea I really like, but i already had in mind that pretty much the world is scared shitless of him, so any divination about him would pretty much just be: "RUN! SCARY!!! BAD!!!"
your idea of the black hole system would pretty much be how his power as a world eater works, but he needs something separate from that so that he doesnt eat everything he attacks.
gravity spells
void spells
space spells
dark vision
energy vision
life and/or soul vision
mana manipulation
space manipulation
void manipulation
gravity manipulation
I believe he/she would have earth manipulation and it variations ex: earth, metal, sand, etc..
he/she might have fire/magma manipulation.
It would be impossible to steal from him/her since the void never gives back what it takes.
this being might not need sleep and since they can eat anything, they could eat air to help with their hunger, they could do the same thing with mana with the added benefit of increasing their mana pool if they have one.
this being may not need to breathe or at least to breathe as often since they are able to live in the void.
since the void is able to eat anything, this would include knowledge. that means that he/she could eat a book and they would know all the information in that book.
this being might have a unique relationship with souls. you can interpret this however you like.
gravity spells = yes
void spells = yes
space spells = main point
dark vision = his eyes work differently so darkness isnt really a thing to him, neither is light. he sees the world by its rules(think something like the matrix except golden and more magical) as well as being able to sense space. Of course he also has human eyes, but they are supplementary and whether they work or not isnt an issue.
energy vision = refer to the above
life and/or soul vision = refer to the above
mana manipulation = thanks to his observation of the laws he can figure out the rules of magic pretty easily, but he needs some help at the beginning
space manipulation = part of space spell
void manipulation = idem
gravity manipulation = idem
he theoretically has an affinity for all elements except for maybe mana because he is technically an amalgamation of worlds, one of which was earth-like so he has all the elements inside him and all the experiences and personalities of every living being, but space is his preferred element and his strongest.
It would be impossible to steal from him cause nobody has better space skills than him, any attempts to break through his item box would be instantly detected and countered. There is no inherent "what is mine is forever mine"unless he absorbs it, but then it is destroyed.
He creates a body that follows the laws of the world to the best of his ability, so he has all the weaknesses and limitations of a mortal body with some exceptions. his body still needs sleep and food, at least at the beginning.
absorbing mana is very easy to him once he figures out how, but he still needs someone to explain the basics of the worlds power system cause he never felt it beforehand.
Eating books for knowledge is a good idea.
He wouldnt have a particular relationship with souls. As a world eater he can absorb anything, but souls would bring the same problem that eating a living person would bring. Souls are just another life-form to him.
I say him even though he technically has no gender (or is all genders and animals and inorganic matter) simply because he takes a male form when he transforms into a human (copies the thief).
What i need is a secondary power system for a world eater that has comparable power (at least by the end of the story) but follows different rules and isnt focused on "absorbing/eating". I want it to be related to space, or at least it is among his possible abilities, but isnt the same as his world eater abilities.