The recent theory/discussion I've seen popping up around the Bleach fandom is that Ichigo's Bankai has the power to "shatter fate".
And I believe that's why Yhwach deemed it too powerful to allow him to use. It would counter the Almighty. Because Yhwach would select a future, it would be "fated," and yet Ichigo's Bankai would "shatter" that fate.
I don't think Ichigo necessarily has the ability to manipulate the future. But rather, it would have the ability to nullify anything that sets the future in stone. And it would return the fate of the future into the hands of anyone around him? Like, it would allow anyone to take their future into their owns hands and craft their own fate.
Basically, it's like the ultimate god-squashing ability, lol. Which is so in-character for Ichigo, if you think about it. He's always taken down people who think they have the right to impose their beliefs or will upon others. Byakuya with his "honour." Aizen who wants to "become a God at all costs." And now Yhwach, who has his own goals and will stop at nothing to achieve them.
I think it suits Ichigo to have an ability that levels the playing field. Allows the "little guy" to have a say in how the future plays out
That's just a theory, tho (a film theory.