The idea behind a "Possess Item -> Next" Combo is simply a combo that will change into another combo based solely on whether or not Link currently has an item with a specified ID.
this would be very similar to any ordinary combo that can be changed into the next listed or specified combo by a weapon trigger, except in this case, the combo changes simply based on whether or not Link has *collected* a specified item.
Some care might need to be taken if a questmaker attempts to set a dropped, finite item as the possession, such as a heart or rupee, but otherwise this could open the way to revolutionizing how Event Flags are simulated in Zelda Classic (rather than, say, extensive use of screen state carryovers.)