I think for 2nd leaders, it is important that their leader screen is very different from the first leader of this civ. Not just the look, but also the feel that is created. Pericles and Gorgo do this nicely.
Bismarck was depicted as an old man with military clothing in civ V. And while he indeed wore things like that, I'd rather have him in civilian clothes - especially if he doesn't get a very militaristic ability. Most photographs of him show him in old age, but that wouldn't be necessary. Being old gives him some more authority in my opinion, however.
I'd actually rather have Stresemann than Bismarck, and if this would ever happen, he would have the wear the clothing style that is named after him of course.
For Louis XIV, I think we all have the same picture in mind when hearing his name.
While Maria Theresia could be said to be more successful, Austria's most iconic leader is surely Franz Josef I. His influence on Austrian culture is also seen everywhere in the country today. If KuK times are remembered, it's always his time. I would prefer if he looked old, but not too old. Firaxis would surely love the beard.
Gilgamesh isn't looking very Sumerian right now, the muscles etc. show much more of an Assyrian influence. If you want, you can see the vague abomination of a Sumerian priest king. I think any 2nd leader for this civ should look more introverted. Like the standing statues of Gudea for example.
What we are lacking in kind of representation right now is an older woman. Maria I of Portugal could fulfill that role, but I'd rather not have her back. Catherine the Great could be like that, too, but again, I'd rather not have her back. Margaret I could be depicted like that, but she would probably not fulfill the cliches most people have of Denmark. On the other hand, the viking thing is already occupied by Norway, making room for a different Denmark. But then again, depictions of her show her as a young woman. Who would be a nice option for an elderly lady leader?