Based on that one image, the humanoids would be awesome to feature in the sequel. Perhaps, they wouldn't be as durable as daddy Shin, and it would allow for some great scenes of them ravaging towns before fighting the military. One of my fears of Shin sequels, is that the Japanese military has ZERO reason to directly attack Shin in a conventional way, in fact it makes him retaliate violently. As a result of a sequel did happen, the humanoids battling ground soldiers would be a great way to have some good military scenes. Maybe even some tension that if they aren't killed quick enough (like within a week), they'll evolve into full sized shins and spread. Perhaps daddy Shin becomes immune to the hydra chemical, but the little guys don't?
Meanwhile Godzilla is growing wings or something, or is getting larger.
If you wanted to throw in some other Kaiju, some bullshit of a classic monster, like Rodan, simply being attracted to Shin's radiation, and beginning to eat the humanoids, is easily do-able.
Then, everything changed when the Giant Condor became the Giant Eagle.