The ability to find offering pieces during trials and craft them later
I would like it if offerings were split into pieces that you could find during the trials. I mean stuff like twigs, flowers, chemicals, etc., that you would find as you played, scattered around the map. Once you had enough supplies, you'd be able to make your own offerings, depending on the materials you had. Higher-rarity offerings would require rare, unique materials.
You could tie some materials to specific Killers (chemicals to the Clown, for instance), but I think that would only work if every Killer was played equally, which we know is not the case.
IMO, it makes little sense that the Entity would offer Survivors and Killers the chance to buy offerings for itself, and it might delay the game a bit if Survivors take the time to look for offering pieces, especially if they want rare ones. Furthermore, players could craft the offerings they actually want, and not have them sit in their inventory forever.