With the PoD at January 1, 1945 at the least. Turn Japan as second only to the US and possibly a second superpower in terms of economy, military, and politics. I assume that for the military, Article 9 would have to be butterflied, or removed due to an unfortunate circumstance. Economically, avoiding the Lost Decade and ensuring continous population growth might be the key.
How can Japan strive to reach this role? And what would be the potential repurcussions of Japan wielding so much power?