Q. What should a new SDK of Miniscript in a Language Contain as features ?

Bitcoin Miniscript implementation in a new Language should contain these features

miniscript parser
type checker
malleability check
script length limit check (P2WSH standardness rule)
op count check (P2WSH consensus rule)
duplicate pubkey check
script generation
witness generation (satisfactions)
unit tests that create receive addresses based on some miniscripts and spend from them again (see TestRedeem()).
Unit tests exist to check that this implementation's type checking passes some of the unit tests of rust-miniscript.
timelock mixing check
stack size check (P2WSH standardness rule)
exhaustive unit tests
witness generation for thresh/multi currently has a naive implementation that is very inefficient (exponential runtime). This should be replace with a fast algorithm.