As I have moved to more fillet work and less lugged frames I too think about the aesthetics of mixing a socketed crown (shell, drop out) with the smooth surfaces of a fillet. One trick I use is to round off the socket points, fill in any non round cut outs and add round holes to the socket surfaces. See above.

Tabbed drop outs allow you to use many different stay/blade ending treatments. I tend to shy away from socket or plug drop outs as I like the freedom to fashion as I please.

There are a few plug type crowns out there. Most are rather heavy, being often solid VS hollow. Not many are for wider rims/tires.

Check out my Flicker albums for various other images. https://www.flickr.com/photos/73195587@N00/albums Andy