Just an idea here, but it would be great to be able to change items while in battle. It should cost you to do this of course, for example I keep a couple of decks handy pre-set for the type of characters I'm fighting. Sometimes I forget to switch back to my main deck after using one of my speciality decks.
If I could change deck duing a battle it would be very helpful. I think it should cost me a round or two to do it though.
I can also see a number of issues with this, that may make it unrealistic. For example this would effectivly bypass the size limits on decks. Also, what about changing clothes, wands, etc... I don't really have an answer to how this would effect deck size limitations, but I think every item you changed would force you to pass for a round. This penalty could prevent people from changing everything during a battle, but it would give a bit more flexibility to the game.