I don't really need a ghost popping up talking to Cloud, he's got enough skeletons and ghosts to deal with as is, but I wouldn't mind little shreds of evidence throughout the world where Zack was essentially taking care of Cloud for a time after their escape from the mansion. Etchings here and there that you can find in obscure areas that don't stand out. maybe a buried note that Zack left behind. Maybe late game Cloud does recognize the area but I think the silent reflection of that space is better than filling it with dialogue.

I also weirdly hope you can get back into Midgar properly this time around. Knowing that Tseng saved letters Aerith wrote for Zack I think it'd be neat to be able to go back in and find them somehow after all the truths have been revealed late game. Just a little more of a glimpse into their relationship and maybe how she gradually moved on over time or something.

I also would find it neat to find some sort of "remains" of Banora. Nothing on the map, just sort of a decimated grove and evidence of buildings no longer there. Just enough to go "oh, so here is where Banora was..."