Hmmm.
I don't think MHA is good enough to lead off. It reminds me of how they tried to force Uno Pacu to be a thing, and it didn't work.
Toonami could stay at the current 10:30 start and continue to do fine, rotating out some of the older shows at the tail end for newer "classics".
I also suspect that Boruto is going to be the next Black Clover, leaving them scratching their head as to where to put it when it just doesn't find an audience in the way that Black Clover just didn't find an audience.
Honestly I'd be waiting to see when the anime for Eden's Zero comes out, and try to snatch that up. It would be the kind of long-running shounen that could find a committed and casual audience and do well when Super ends.