These are things to consider making mandatory in the large language that are optional in the small language. See WG2Dockets for other dockets.
Full numeric tower support (bignums, ratios, flonums, compnums)
Forbid mutation of literals
Exact complex numbers
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 downcasing of Σ.
Support multiple inexact-number precisions