Back in 2010, there was a bug found in Core which allowed denial-of-service
attacks due to the software crashing on some machines while executing a
script - see CVE-2010-537.
I believe the removed ("disabled") opcodes should be re-introduced along
with a standardized behavior definition.
For example, when execution of an opcode results in an arithmetic error,
such as OP_DIV with a zero divisor, the script should exit and fail.
The string splice opcodes should also check their arguments for
correctness, etc.

These opcodes would enhance the flexibility of scripts and allow
sophisticated native smart contracts to be created.