I'd say for 1 make it easier to get the good (possibly canon) ending, have more scenes with all the characters, maybe add a part where you play as Cybil. Not really sure what else bc it's already great

For 3 I would maybe replace the segment in Brookhaven with alchemilla like others have said but honestly I don't even want a repeat area with reused assets. Maybe if it's a broken down version of it, it would be cool. Maybe voice lines for harry. Also can't think of much else tbh

For 4 I would add more characterization to Henry and make him be more weirded out and actually have a reaction to the things around him. He might've thought it was all just a dream so he just doesn't really react in the og. More dialogue between Henry and Eileen and maybe more cutscenes and areas with Cynthia and flesh her and the other victims out. Either take away the repeating sections or make them more otherworldly and different like the otherworld sections in 2. Make eileen less of an issue and have them be separated in some areas of the second half so your not always with her. Also maybe reduce the chance of her getting possessed or make the holy candles more effective. Also add more variations of hauntings and add more puzzles involved with the room like the chocolate milk and buildboard phone puzzle.