More BASIC?

[Tom Lake]

Are there any 3rd party BASICs for the 680 that are more powerful than the 8K Altair BASIC?

I'd look for features such as the ELSE keyword, PRINT USING, maybe double precision.

[Cliff Chism]

Might not be of any help and it might be out of your wheelhouse, but Vol 1 of Dr Dobbs Journal (1976 and available on archive.org) starts a series of articles on writing your own BASIC that will do everything you ever wanted, if you so desire.