For cryptographic hashes,

• it is infeasible to find a message that has a given hash,
• it is infeasible to modify a message without changing its hash, and
• it is infeasible to find two different messages with the same hash.

Your approach isn't likely to help the second and third feature I mentioned.

For example, the successful attacks against MD5 created two messages of the same length with the same signature. Your variation is no more secure than plain MD5 in that case.