I think a more tender portrayal of Sweeney could work, if it was done with deliberate direction. After all, he WAS a normal innocent man who was driven to his actions through grief. If a director played up a sense of insecurity, of sad desperation, of manic brokenness, I could see it working.
The problem is, everyone has this idea in their heads of who Sweeney is, so actors will play into that archetype even if they aren’t quite right for it. And directors don’t challenge it, because they also have a firm idea of how Sweeney should be played.
Will Kail be the one to break that mold? I’m not sure. I’ve seen 6 shows directed by Kail, and frankly I’ve never been able to nail down any of his stylistic hallmarks. And I’ve never seen him direct a revival. However, one thing I HAVE noticed about his work: it tends to be deeply felt. I think he’s demonstrated a consistent sense of tenderness, and emotional vulnerability. If he carries that into Sweeney, he could be the perfect person to help Groban pull off a more “soft” Sweeney.
But honestly, as long as the technical acting is there, I don’t think I’d mind too much if his temperament doesn’t entirely fit the character.