Welcome to hell, anon. Imagine that the ending has already been completed and someone has already seen it.
You may now watch the anime and movie and pretend that one of those endings is how the manga would end. Or you can make your own.
What are your fave characters from TB/X?
If you want more vintage Clamp feels, I highly recommend Clover (apparently it also was never finished, but you can't tell from the story) - IMHO it has the most wonderful art in all of Clamp's manga or RG Veda if you would like to read a proto-X with an actual ending.
Shirahime Syo is a wonderful oneshot as well.