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)
Exact complex numbers
Return exact results on exact arguments whenever possible (e.g. (sqrt 4))
Full Unicode support (all Unicode characters as characters, all Unicode characters except perhaps U+0000 in strings)
Fixnums must be at least 24 bits signed
Signal an error on all ItIsAnError situations
Provide all R7RS-small libraries
Forcing a non-promise returns the object
Implicit forcing of promises by primitives
Require Unicodely-correct downcasing of Σ.