One of the draw backs of an MP5 is the trigger. The triggers are long and heavy. There are some people who claim to perform trigger jobs but the results are mixed. Marek Marekovič has been working on a custom lower for his MP5. It allows the use of a drop in AR-15 trigger.
Here is a test video he posted.
Why hasn’t this been done before? Well it has been attempted. Bretheren Arms posted on their website that they are working on one back in January.
There there is Guy In A Garage who has been documenting his attempt to put an AR trigger into his Zenith MP5.
One of the biggest challenges lies with the ejector. It is a part of the trigger pack so if you want an AR15 style trigger, you need to add a new ejector somewhere.
Guy In A Garage shows off his ejector in his second video.