I also have a 1200gs and am looking at a second bike for the same reasons you are. I have had many bikes over the years and to find one bike to do it all is pretty difficult. I,ve had Ktm 950 adv. ,640lc4e,525exc 250exc,200exc,.I had a Yamaha Wr250r and a Kaw klr and a few more. All great bikes in there own right..Of all the bikes I mentioned I found the Ktm 640lc4e to be the one closest to what I am looking for now.So that being said I will be looking at a used Husqvarna TE610 or TE630 and possibly the new Husky 650 Terra come spring, although I think that the Terra may be a little heavier than I want but maybe a great bike for you . I think the mid size adventure platform (650cc) is the best compromise for what you may want .It will go on the interstate at a pretty good clip and is capable of doing so decent off -roading ,dependent on your abilities.They also have a enough power to put a grin on your face on the gravel backroads.This is just one old man's opinion and I hope it helps.
Good luck on your choice.