The end of season 5a saw Rick involved in very questionable form, violent and amoral. Carl barely spoke but seems determined to fight for the weaker but is still maturing. He may veer toward violent instability again.
Carl is Rick's off-switch and visa versa. It's their story so both cannot be lost to bloodlust together or that cycle couldn't be broken. Rick needs to change and Carl is vital to all chances of that happening.
Either Carl will do something horrific, which Rick will see as his fault, snapping him out of his amorality to save Carl or Rick will commit a crime so heinous as to alienate Carl and his fear of losing his love will change him.
The list of crimes that could shock either of those two like that is quite short at this stage. Rick will not keep slaves, eat or rob strangers and neither will Carl. It will have to be a killing with some emotional impact.
(My suggestion is either Carl or Rick will kill one of the group as we know them by the end of
Season 5b. The only crimes now are against your clan. Abraham and Rick will butt heads and during that fight an Abraham shaped corpse would have the required effect.)
Or something else...but their interactions will define 5b because Rick better come back. Ricks not home right now.