I heard that Taiwan actually had a bus route 0, but whether that bus route is still around or not, I'm not too sure. It also seems to be the only example where the number 0 was used alone for a bus route number anywhere in the world.
I once read in the Newman magazine a small column where they asked SBS Transit how they get numbers for their bus routes. It was explained that bus routes starting from one area try to use one specific number system (example: 5X bus routes mostly start from/terminate at Bishan), and SBS Transit further said that they can use any number from 1 to 499 (fancy a route 444?

).
As for the number 0 itself, I don't think it will ever be used, even 100, 200, 300 years into the future (God knows how the bus system will have evolved to then

), when more parts of Singapore are developed and more bus routes added. At the most, they might use letters more liberally (examples that can be seen today: CT28, C3, and NR1), or they might resort to using four digits (imagine bus routes with numbers like 8888 or 1748 or 9413 or even worse, 4444

). I still remember that many, many years back (early 1990s, I estimate), I dreamt of an old NAC Nissan U31SCN (not the ones SMRT are using now) whose desto beared some strange route "code" consisting of numbers and letters - can't remember exactly the sequence, but it was something like this: "N30SR517". This might be another possible alternative should SBS Transit or SMRT run out of route numbers to use.