How to fix the fidelius charm
The fidelius charm is one of my biggest frustrations in canon. It's too powerful. So I decided to to see if I could tweak it so that it serves the same narrative functions it does in canon while not being so overwhelmingly powerful.
The first thing I would change is that I would make it explicit that the fidelius is tied to locations. This is assumed, but never made explicit. Flitwick's explanation of how the charm functions makes it sound as though it works for any secret. It's pretty minor, but it's still a nice clarification.
Second, and the most important fix, is that I would make it impossible to be your own secret keeper. Find some other way for the trio to hide in shell cottage, or in another place. But remove that. It's totally stupid. While I assume that the charm has been refined in the years since the Potter's deaths, since otherwise it makes them look completely stupid, we actually don't know if this is the case. I don't think we see any other cases of the spells being improved after creation, so there is evidence against it in this case. Nevertheless, even assuming that the charm was improved, the fact that you can be your own Secret Keeper is stupidly powerful. As long as you know the fidelius, you can never be found, no matter where you are. It's a perfect defense. So I'd remove that.
I'd also make it so that the Secret Keeper can only stay in the Secret location for a limited time. Say one or two hours. The exact time doesn't matter, what matters is the mechanism. The longer a Secret Keeper stays in a location, the more stress is put on the charm, until at some point it breaks. It takes time to cool off after the Secret Keeper leaves. This would allow for things like Dumbledore's meetings in OOtP to still happen, while keeping the power of the charm manageable.
I would also make it so that you can only be Secret Keeper of one location once. So James and Lily refusing Dumbledore's offer of being Secret Keeper would be motivated by a desire for him to still be able to be Secret Keeper of other, more important locations, rather than the idiotic move it was in canon.
Another change is that there can be more than one original Secret Keeper. As is, if the Secret Keeper dies, everyone in on the Secret becomes a Secret Keeper. By making more than one Secret Keeper, if one of them dies then the other living Secret Keepers can still keep the Secret, and it would require the death of all of them for all the people in on the Secret to become Secret Keepers. This would make Sirius's plan to switch Secret Keepers as a decoy make more sense. He would still have been a Secret Keeper, but not the only one. Peter would be too, but in secret. Everyone would reasonably assume he was the Secret Keeper, and would try to kill him. By killing him, James, Lily and Harry would become Secret Keepers, and would have to leave Godric's Hollow in an hour. However, if Peter was also a Secret Keeper, then that doesn't happen. He's a safeguard. According to Sirius, nobody would believe that they'd dare make him Secret Keeper, so nobody would target him, and the Potter's would be safe even if Sirius died. Of course, Peter betraying them would throw all of that into the trashcan.
I believe these changes would serve to improve the story, and would be an improvement. I would appreciate feedback!
