These are things to consider making mandatory in the large language that are optional in the small language or in SRFIs that form part of the large language. See WG2Dockets for other dockets.
Full numeric tower support: arbitrary rationals and inexact complex numbers
Forbid mutation of literals
Exact complex numbers: not required
Ephemeron-based hash tables
IEEE 64-bit binary inexact numbers
Return exact results on exact arguments whenever possible (e.g. (sqrt 4))
Full Unicode support:
Fixnums must be at least 24 bits signed
Signal an error on all ItIsAnError situations
Provide all R7RS-small libraries:
Raise exceptions on all errors
Forcing a non-promise returns the object
Implicit forcing of promises by primitives
Require Unicodely-correct (but not AI-correct) downcasing of Σ.
Support multiple inexact-number precisions