You realize you are truly trying to sap all the fun out of the card right? Do you like monotonous play? And it is a spell because making it a permanent means it can be removed, and there are plenty of permanents as it is, and you would only ever need one out, but you would need more than one in the deck for the purpose of increased probability of drawing it. As a spell, I could see more people using it, as a permanent, with not a fantastic ability, but a cool one, won't get used as much because it is easily removed.
People tend to ascribe their own abilities to others when they talk. Since I am ascribing the ability to see relevant information, cut out extraneous detail and memorize a subset of the data as a matter of habit you might want to assume that it is easily possible if not necessarily easy to do (although it is not hard and develops naturally as you gain experience in the game or related games).
Okay, that's you, that is not everyone. People will have to start paying attention a bit more if they know their opponent might be using the card, good.
So there really isn't anything BAD about the card, nothing making it too good, or completely useless, you're more or less drawing straws making up reasons that really make no sense. You've also not mentioned the cost of the card or wording or anything else, just the effect seems bad to you. But from what I've gathered, you probably like to play rush decks since they tend to win more, and are fast. Broaden your view for a bit, make sure to include a little fun in your decks, even though you have to grind and grind to make money, there is no reason not to include fun cards into the game of elements just because they are fun. This is a game after all.
Let me try to rephrase my comments sequence:
Currently it only has an effect if the opponent cannot identify the field when given an opportunity.
Currently it has the good idea of changing the appearance of creatures to add secrecy.
These two details means that it is an incomplete card as of now because it does not accomplish its goal against a quick opponent.
My two solutions for this problem have been:
Make illusion an ability of a creature. (you decided this is not worth considering.)
Change the mass illusion effect to predate the playing of the creatures it is meant to hide. (This idea you did not understand. Likely because of how I phrased it so I will outline it)
Turn 1:
Play Illusionary Veil. It has the number 1 on it.
End turn
Illusionary Veil decreases from 1 to 0.
Turn 2:
Activate Illusionary Veil's ability "All creatures played this turn look like target creature."
Play creatures (Note: Your opponent will still see the order of creatures played unless that is coded into the effect of Illusionary Veil. Since the Veil would be in play it is possible for it to affect the card playing animation if Zanz wishes.)
End Turn
Illusionary Veil is destroyed. Creatures remain illusioned.
To answer you hostile sounding questions:
I am trying to make your idea work even against people like me. That is adding fun not sapping it.
I hate monotonous play.
Yes as a permanent there will be a 1 turn vulnerability to Permanent Control. However as I better explained above the illusion is created by the permanent but does not need the permanent to sustain it.
Like Sundial it would be a permanent that is used like a spell.
One turn of vulnerability would not deter its usage.
If what I said doesn't make sense, perhaps you should read it again. I tend to know what I am talking about but I write as if the other person is also well informed.
I have not mentioned the cost of the card because a mechanic has to work before it can be evaluated. Once it works then the next step is identifying a balanced cost. I also tend to not comment on balance unless it is unbalanced or I suspect evidence of its balance would be useful to provide myself.
You jump to conclusions too fast. Here would have been a better series of intuitional jumps about my in game character:
I can evaluate the field quickly
A more complex field exaggerates my advantage (If I were playing only to win. Instead I prefer a fun deck that is moderately good at winning.)
A more complex field would be created by a rainbow stall deck not a rush deck.