Fair warning: I'm pretty old-school about things like Wishes, YMMV.
The limitations listed under the Wish spell are suggestions. Wish is a powerful spell, and should be able to do powerful things. On the other hand, the key to using a Wish successfully is not to get too greedy... Anything on that list you should be able to get away with, without much in the way of repercussions, under normal circumstances. There are always exceptions, however...
Now, assuming normal campaign conditions I don't think that there should be any problem with a Wish spell delivering any one magic item with a sale value of up to 25,000 gp, or adding properties onto any item such that it's total cost does not exceed the same amount. This assumes that the item in question is one that a character (and by this I mean a 17th or higher level Wizard, even if you're some low-level schmuck who just happened to find a Ring of Wishes...) could reasonably obtain. If the item is something that only one cabal of evil Wizards can produce, but they are manufactured in numbers and at least some of them are owned by, say 14th level or lower NPCs, then no problem. However, if it's a specific magical key that happens to be in someone's possession (ie: not just lying around in a dungeon somewhere), then I'd be very wary about how I worded my wish... Likewise, in a campaign where magic items are rare, expect to get away with less.
Something, specifically, about adding powers to an existing magic item: you could probably just add 25,000 gp of worth onto most items. The one specific place I'd expect there to be an exception would be items that are worn in a specific "slot". Adding extra powers to such an item lets you get around the rule against "doubling up" items, and I'd expect such a thing to get a close scrutiny by the DM.
And keep in mind that the DM is under no specific obligation to mess with you if you ask for above and beyond what is on that list. If it's reasonable, or even just cool, you can get away with a lot. Like I said, just don't get too greedy. It may often be best to phrase the Wish in such a way that it is open-ended such that the DM can give you as much as he thinks you deserve (or can handle). In all fairness I think it's the DM's responsibility to make your Wish fit within the bounds of the listed effects if in any way possible before just outright screwing the characters...