To my knowledge, the only boundaries set upon a lucid dream are your own knowledge and your own perception of impossibilities.

You wouldn't be able to, say, learn a real language in a dream, unless you already understood it - but you may be able to create one.

And if you do not believe it possible, you cannot happen.

Another limit, set by reality and not a dream, is of course time - how long can you dream for, anyways? But even time is a matter of perception.

Anything that would be an exception to this...well, let's not get into speculation and superstition.