Would it be possible to add ~just/~left patterns for the container types
from SRFI 189? I think that would be helpful for error handling. Also a
match pattern to deconstruct record types could be useful.

I think the dependency on SRFI 115 is OK. Match patterns for data
structures from other SRFIs would also be helpful to include. So long as
the SRFI that is depended on is implementable in R5RS/R7RS, like this
SRFI is.