The basic idea for my first design is an m4 with an underslung shotgun, with everything meshed together into one gun. It would require some modifying to the receiver and main body of the gun, and total redesign of the front end.
As you can see in the picture, it almost looks like a SCAR. The split level top rail would be for making it easier to mount a scope or sight on the back without the attachments in front blocking it, such as a PEQ. The entire shotgun is to be part of the front end.
The dust cover on the side is part of the modded body section with the release for it being moved to the side instead of the back, although moving that might be more work than anything else.
Flaws:
-I know I drew a straight mag
-I didn't include any iron sights
-Not sure how the shotgun would be loaded until I had one to work with.
EDIT: That blank space above the mag would be for my own engraving, I left it blank cause I don't want to think of something at the moment.