Seeing as this is an interesting ability with a reasonable amount of design space how should it be balanced? As is its not balancable. Drop the p/t bonus? Add a cost? Both options are interesting but their are even more points to consider. Should this exile or be reusable?
The way its works I see it as Shaman King style oversouls. However I don't think an execution that captures that is reasonable. Instead I'll initially opt for a kicker like extra cost and make it a once time use.
Sword Saint 1WW
Creature - Human Warrior
First Strike, Vigilance, Lifelink
Oversoul(Whenever a creature enters the battlefield under your control you may pay ~'s mana cost. If you do, exile ~ from your graveyard. That creature gains all abilities of ~ and gets +X/+Y where X is ~'s power and Y is its toughness.)
Is this too restrained? Should the cost be uncoupled from the cards cost? Should it be reusable? Is it overly complicated?
Only it's doesn't need a cost—because you can't build a winning strategy at the beginning of the game with the notion pitched due to the probability factor. Probability is a significant aspect in development. It will see things succeed whose properties succeed within its natural container. You will not have to adjust anything. But more importantly, will see things fail unless implemented around properly. I have given a graphic example of this in the past, with the set of lands that only work when the range is increased to allow 6 copies of them in the deck.
Now, I am not against revision of the keyword, or that a cost could be assigned to the functionality. I just want to make a clear point that this is not needed; as neither is the Aura type. In fact, adding that type could botch the flavor-dynamics, and opens up functionality that this doesn't want to have; but wants to be more solid state than this would create.
There is so much wrong here that I can only conclude that you don't know what most of the words you used mean. Because you are obviously lacking is comprehension skills ill try to be as simple as possible.
This ability isn't bad, but it is unreasonably difficult to balance because you didn't put a cost in its activation. Your proposed card while strange isn't a problem because spending multiple resources to get a few 'free' +3/+3 auras on a creature isn't significant. However, the ability is a problem because getting a few +5/+5 auras or anything higher is significant. This is why I said you either make it cost something or you are forced by the design of the ability to only put this on small creatures.
Your lack of understanding or unwillingness to accept that your design has obvious problems can't be handwaved away by using words you obviously don't know the meaning of. Fortunately, others here see that this is an interesting ability and can engage in a discussion of its pros and cons. Honestly, it would do your design a disservice for you to continue in the discussion as you don't understand how the game works and actively discourage people from fixing your mistakes.
Comp rules easily hammers out the technicalities of this card being attached to another card (without ever having to give it another type); and can also easily rule that a card attached this way is exiled if removed or otherwise would be put into a graveyard.
They also can solve this much more easily by giving it the aura type so that is what will happen. I know you feel you can hand wave away any problem by saying the rules can be rewritten to handle it. However, a good designer has to ask themselves(or someone who knows how to the rules work) "How can what I want to do be done in the current framework" then They ask "is there a better way to do this". Only a bad designer starts with "the rules will bend around my idea."
Since there is only like an 8% chance of pulling that combo with Pocket Hulks in your opening hand to perform on turn 1, I don't think it was worth addressing. Even with just 1 Hulk the chance is still like 12% or something. You could add lots of other 0 cost creatures, draw/discard, but you're still not building a game winning strategy from that. It still dies to removal, and Lightning Greaves will extend the time range, and plummet the probability even lower.
There are many problems with this mindset and none of them are related to your actual card.
The problem is the mechanic itself. It needs a cost or the problem they pointed out will occur because you are proposing a mechanic, not a single card. meaning there will be a number of cards with this ability and it would be easy to discard a few on turn 1 or 2 and get a huge creature for "free". Or you create the limitations of these being no bigger than 2/2 so thst isn't a problem.