It owns close to 80% of the Japanese ETF market currently, with no end to the expansion of balance sheet in sight.
After buying long treasuries, it's not unreasonable to imagine the Fed buying stocks, either individual issues or ETFs. The President would be for it, and it would be hard to drum up any opposition to it in congress.
Not only that, but rates on long treasuries have stayed negative for long periods of time in other countries. It's an open question how negative long treasuries can go, but the evidence suggests we're not even close to the limit.
Recessions are politically unacceptable in today's world. Buy stocks. Buy treasuries. Do so with wild abandon, because the Fed has your back. Just watch out for the moment when the whole thing jumps into reverse and no amount of market manipulation will stop the bleeding.