JDelekto: I would banish the word 'the'. It is ranked #1 as the most used word in the English language; and as such, both ingrained in writing and speaking, would require bit of thought on a suitable replacement.
My purpose for doing so would be to see how creative people can be without it.
Leroux: Why would you need a replacement at all? Just skip it. I actually think that wouldn't be such a big change, once you get used to it. (At least not if you still keep "a".)
It's funny you mention that, because 'a' was the first thing that came to mind until I decided to Google it, after all it is a letter and banning a letter, much less a vowel. The last thing I would want to have is uncontrollable vowel movements.
So, what I ended up with was a word which was apparently most used.
Now, what if we started paring down the the "most used" words so one was required to be a bit more resourceful and use a suitable replacement?
Not sure if that's really what we want either, because that's probably what happens to movie scripts created from great books.