You could include a hash of the correct scriptsig (only the static bits) that is required in the txn ?
But how do you decide which scriptSig is "correct"? This is one of the challenges the Bitcoin developers are currently facing - attempting to define a set of rules to make sure only one version of a scriptSig is considered "correct".
When Bitcoin had the tx malleability problems people said it was Bitcoin feature and any problems were the exchange's faults for not writing their software correctly. I think most Bitcoin exchanges modified their software afterwards to prevent problems. Did all the alt coin exchanges modify their software to prevent any problems?
I've seen people justify many of Bitcoin's deficiencies as features, but that's a discussion for another place and time. There are some benefits to non-malleable transactions besides being more robust against bad/uneducated programmers (which by itself is a good enough reason, IMHO). For example you can create transactions that use the outputs of another transaction before that transaction is signed.