I’m trying to think of interesting ways to make the Healer feat more relevant in the later levels. I’ve considered higher quality healer kits to add more d6s, as well as incorporating various flora that can be scavenged to remedy conditions or provide buffs.
I think it's important to address why you want to address some of those qualities. If you're looking to remedy conditions, is it because you want something mundane that can be used in place of magical healing? Or do you want it to supplement existing magical healing in a party? It's important to consider exactly what you're trying to replace or supplement within the existing mechanics in terms of how your table plays before you can start adding to them. If you have a Cleric at the table that enjoys being the one people can rely on in the case a Restoration spell is needed, it may not be a good idea to offer other players a single feat that can replace their uniqueness. If nobody wants to be forced to pick a class with such options, so you have no such characters at the table, it may be a good idea to literally just add the effects of such spells if the party truly needs them.
A new idea I just thought of would be to eliminate the healing portion of the feat altogether, keeping the 1 hp stabilize action, and instead allow a use of a kit charge during a short rest to maximize hit dice recovery. This obsoletes the Durable feat, but I never see anyone take that anyway.
Thoughts?
Personally I feel the Healer feat is intended to keep the party going a bit longer between combats before they need to perform a short rest, so that's along the lines of what I'd try to stick with if I needed to rebalance it. As mentioned by Ashrym, the fact that you never see Durable is an indicator it's one that needs to be reviewed, so maybe you can adjust Healer to give such feats/abilities more power without stepping on their toes.
Maybe have the Healer feat allow people to recover Hit Dice without having to take a short rest. For example, they heal the normal 1d6+4+1/HD, but it also allows them to spend some number of their Hit Dice to recover more when that part of the feat is used. This would allow a person with the Healer feat to provide additional bursts of healing during dungeon delves where the party may not be able to take an actual short rest, while not overpowering the amount of healing that can be performed throughout a day that provides more readily available short rests.
If this seems overpowered in your combats, perhaps impose a limit on the amount of HD that can be spent if you're not able to spend at least 1 minute tending their wounds, or perhaps go with feats like Inspiring Leader or spells like Prayer of Healing and just require 10 minutes of downtime for the party to do it. You could even add some relevance to the Medicine skill and make it so that a player cannot grant more Hit Dice per use than points they have in Medicine.
If all of this seems under or overpowered because your table provides short rests whenever they're needed, perhaps grant it the ability to restore Hit Dice that players have already spent, allowing it to boost their natural healing throughout the day by increasing the amount of Hit Dice that can be spent between long rests. Many people seem to forget you only get half of your Hit Dice back per long rest, so this could be quite a boon to higher level characters. This could go with the same types of limitations if it proves to be overbearing, like being limited in some way by the Medicine skill, or just putting some type of solid cap based on level.
There's a million and one ways one could probably balance mundane healing, so this is just some food for thought.
