Currently, TB and FB allow bases up to 36. This is because I'm lazy and used Python's int base conversion for FB, despite the fact that there's no equivalent in the opposite direction and so I had to implement TB from scratch anyway. If both were implemented from scratch, we could support custom bases up to at least 62 (using digits 0-9A-Za-z) and probably 96 or higher (picking some arbitrary order for the non-alphanumeric characters).

It might also be worthwhile to add a new operator that mimics Python's int base conversion, in case its less-permissive behavior comes in handy in some situations.