Or do something curvilinear?

The round windows are sort of out of place so lanterns would be better but I would not make them too prominent.
Be aware that the chimney will likely need major adjustments to pass code, I know that this is just a rough image but it is something to think about.

Athome, thanks. Do you prefer one shape over another.

ANd you hit on the main issue with my mock up --- the new dormer will not necessitate any change in chimney height.

I prefer the second and it is good that the chimney can stay as is.

I've been researching outdoor lighting for another project, and have been pining for this... It would mimic the window in your mock-up.
http://www.barnlightelectric.com/outdoor-decor/barn-post-lights/charleston-barn-lantern.html

If you're interested in a nod to the nautical, this source has great designs. I just have a difficult time determining what's rated for outdoor use from their site.
http://www.capecodlanterns.com/

Here is a link that might be useful: Charleston Barn Lantern

Thanks, Peony, that is a very interesting one. I think I should try some mock ups.

I would wait until the upper dormer is finished before deciding to do anything there. As pictured the third tier or dormers is almost all white woodwork, and I would be afraid to lose too much of the shingle area that defines both the style and that wonderful gambrel shape.