For walk and shoot remember you will do an awful lot of walking and relatively little shooting. (And that walking probably won't be over the easiest of terrain).
So, I would say lightness is the key, as (maybe you're different) I find when I'm shooting "walked - up" with or with out dogs, the gun spends a lot of time in my hands rather than over my shoulder so I like 'em light.
So, my ideal gun would be:
O/U (I like being to choose relatively looser or tighter patterns)
Single selective trigger
Nice broad sling
Perhaps modify the ejectors to extractors so they don't throw spents and you have to go guddling under bushes for 'em.
I'm still looking for my perfect walking gun... one day