When will Shiro return to a focus banner?

It's impossible to know this far ahead of time, but given that he comes with two great skills (steady stance and defense tactic) it's likely he'll reappear on a "Heroes with stance skills" or "Heroes with tactic skills" banner in the future. Stay strong!

I'd love to know as well. He's basically a more expensive Lukas.
But only time will tell with him being a focus.