Going back to the original topic..:
I would say that the clear intent of the text in question ("Enchanting Items" on pages 198-199) is only to allow a character to
1) transform a "plain" magic item into one [an existing one] with a property [property or power]. The example given is turning a +1 Longsword into a +1 Flaming Longsword.
2) "upgrade a magic item to another item with similar properties", with the DM's permission. The example given is turning a thundering bow to a thunderburst bow.
The original post of Orcus Porcus, the original poster (the OP of OP, the OP?) suggested a third use, adding a property to an item that had none. The example given was using AV items, so I'll use the first combination suggested by the PHB (one of them at least): taking Leather or Hide Bloodcut Armor and adding the property of Darkleaf Armor to it.
On the surface, this satisfies all requirements of the ritual (as expanded by AV).
Personally, I don't buy this. Apart from common sense, I have two specific rules-lawyery objections, somewhat related:
1) This creates new items, specifically items with a level of "undefined". This should clue you in to this usage being unintended.
2) As I see it, the text uses "property" in a loose way, referring not to specific headers of item descriptions, but to things like "flaming" or "lifedrinker". If you agree, this means AV does not allow the ritual for the suggested use, because it does not create a defined property.
But if I were to allow this, there is no guidelines on assigning a level to these items whatsoever. So (again with a big if) the natural reaction would be not to change the level.
This means that AV (hypothetically) allows Bloodcut Armor with the following line added to it:
Property: Gain a +2 item bonus to AC against the first attack
made against you in each encounter.
(taken from Darkleaf Armor)
Now, playing the devil's advocate: can you find the rules language that says this isn't Bloodcut Armor?
Because if you can't, a case can be made that "the ritual can also be
used to place a property in a magic item that has no property" indeed...
...for no increase in level, thus no monetary cost (other than the usual fee for an item of that level), thus we have power creep.
And with this we have arrived at the possibility suggested by the OP!
Now, I believe I have shot down this theory with the above objections, but feel free to add your comments anyway.
One note though: I believe one reason why we have felt as if we are talking beside each other is because some of you seem to assume avoiding power creep is simply a case of inventing an appropriate level for new items. While in practice you're right of course (and as a fellow DM, I encourage such practice) now we're dealing in theory.
Yes, rules lawyering. And there is no such thing as "assigning levels to new items". There are no rules for creating new items at all. Either an item has been defined by the rules, or it hasn't. Either an item has a level, or it remains undefined. By this I only mean that the argument "you avoid power-creep by changing levels of items" has no basis in the rules as written.
At this time, it is perhaps prudent to point out the fact I'm basing my objections to the OP's suggestion wholly without such recourse, completely basing my argumentation on actual rules text. I ask you to do the same, or the topic quickly becomes subjective, and thus pointless.
Of course, feel free to point out any errors and inconsistencies I might have made.