Though quirky, you made a great suggestion IMO.
Yes. It’s exactly what SICP’s interleave-delayed does BTW, from the section on logic programming, of course 
. I agree its looks are somewhat peculiar., but interleaving streams is an advanced concept anyway, so who cares?
We don’t have to wait for something like that having been implemented in vanilla Snap!, do we? (It could be changed in a later update of the library)
These options have defaults, so the user may ignore them.
As for terminate stream (it’s really a variant of keep, avoiding infinite search, in applicable circumstances): I can both think of cases where you’d want to see the pivotal item in the result, and where you don’t - and if there’s going to be further processing done within the streams domain, inserting an operation just for filtering out a single item is disproportionately “expensive”.
Do you have any news on upvars a/o ring parameters?