Interesting. The use of double stops, whether with an open string or two fretted strings really fills up a tune. The question is ... how do I get there from here? First off for me, I practice scales everyday - different scale types, different keys. Then I see where adjacent fingers - in those scales - will give me a harmonic voice. That's a start.
Another really interesting way to explore potential voices just crossed my desk. John McGann has a tremendous book on Exploring Melodic Variations on Mandolin. This book is a good, fun, year long study of some very common fiddle tunes. And what to do with them. (Scroll down to the second listing on the site posted above.)
This is a very comprehensive study - with the tune / theme followed with four variations both in notation and tab ... on each page. (Some tunes take over 8 pages to explore.)It's worth the price of admission and - it will introduce you to a ton of ideas between vanilla and complex.
Mandola fever is permanent.