Super Saiyan God quite the misnomer wouldn't you say? When it doesn't have anything to do with the original Super Saiyan line of transformations. It's just a Saiyan that achieved the use of god ki. Now Super Saiyan Blue or as it was originally tongue twistingly known as Super Saiyan God Super Saiyan, its just as you said, God ki with the Super Saiyan transformation added to it.
I don't foresee us every seeing SSB2 or SSB3, as of a 2014 interview with Toriyama, he stated that all transformations in the Super Saiyan line past the original form are just powered up mutations of Super Saiyan. He implied that given enough training a Saiyan can boost their base form and Super Saiyan transformation enough that they wouldn't need to bother with Super Saiyan 2 or Super Saiyan 3. My opinion is that he's taking the same logic with those transformations like he did with Super Saiyan Grade 2 and Grade 3. They're stress powered up versions of Super Saiyan. Super Saiyan Grade 4 aka Mastered Super Saiyan is what he had Goku and Gohan strive to achieve.
It was even implied in Dragon Ball Super that Goku and Vegeta pretty much achieved the same thing with Blue over the course of the series until now. Notice how the stamina drain issue became less prevalent? They trained through it and perfected the Super Saiyan Blue transformation. If I'm not mistaken when they fight Broly like this....
They're supposed to be at Perfected Super Saiyan Blue. The movie seemed to follow the manga canon which is why we didn't see Vegeta's Super Saiyan Blue Evolution (Super Saiyan God Shinka) or Goku utilize Super Saiyan Blue Kaioken. Neither happened in the manga and I'm okay with it. They're flashy and good for animation but in terms of story telling they do very little.
On a final note, Super Saiyan 4 is a completely different transformation line than the aforementioned two branches. Its also potentially just as powerful as SSB with none of the drawbacks a mortal experiences utilizing god ki. Super Saiyan 4 for President! *Mic drop*