Give health vials a chance to be replaced by this mod's items, each with differing chances, i.e. green herbs having a 45% chance to spawn while red herbs have a 15% chance. My suggestion of chances would probably be as follows:

• Green Herb - 35%
  
• Red Herb - 15%
  
• Blue Herb - 10%
  
• Other (mobile crafting, extra space, cure) - 5%

For the people who like to play campaign or horror maps or saves, this could probably enhance the experience even further than just the inventory.