There is a forum here on The Dis about creative ideas. Can't remember what it's called off the top of my head, but you can get a lot of ideas to Disneyfy regular clothing.
The easiest way would be a "hidden Mickey" design appliqued or transferred onto the back of shirts. Very gender-neutral and colours can be varied by personal preference.
Beyond that, I would suggest packing just regular clothing and going off site to purchase some inexpensive Disney shirts, caps, etc. And save just a bit for a special clothing item purchased on site that can be worn and enjoyed for many years in the future. Depending on the item, expect something in the area of $50 to $100 on site. Obviously, some things would be less expensive, but even t-shirts tend to be around $30 each or so. Most of the inexpensive items tend to be made inexpensively and tend not to hold up well through extensive laundering. That said, sometimes you can score an incredible bargain either on site or off site!
Be sure to leave space in your cases and save a portion of your luggage allowance for your new purchases.
I know some people in the UK who bring two or three days of clothing with them (their cases are almost empty), then purchase everything else in the Orlando area--particularly a couple of gentlemen with very large feet (by UK manufacturing standards) who have difficulty finding trainers in their size in the UK! *LOL* Most people who do this also purchase additional luggage allowance for each person. Not sure if that works the same for Australian/US flights.