Two small improvements to the N64 Castlevanias would go a long way.
First: Directing saves to cartridge, instead of controller pak.
Because N64 controller paks are awful at holding saves. They depend on volatile memory using 20 year old batteries that are soldered to the board, and not replaceable.
Also, after market controller paks are easily corrupted - making them just as bad. Sometimes, FRAM modded controller paks pop-up on eBay for 50 bucks, but that's probably the most expensive 256kb in existence - if they're even available.
Don't cartridges use batteries too?
Yes. But when playing N64 ROMs with flash carts on real hardware, all cartridge saves get directed to the SD card. However, if a game saves to controller pak, there's no way around it. You must use a controller pak.
So why not just emulate it?
Load up Legacy Of Darkness in an emulator, and compare it to real hardware. Holy crap...
So why assume it would be a small and easy hack?
Because the Japanese versions of these games already save to cartridge. I assume it would require swapping a few lines of hex from one version to another. But I'm not that knowledgeable. Feel free to correct me.
Second: Making all characters playable from the start in Legacy Of Darkness.
Originally, they're unlocked by completing and saving the game. And I can't tell you how many times I lost a completed save because of a damn controller pak.
The N64 Castlevanias are the only games I know that save to controller pak in the US, and cartridge in another region. But if there are other N64 games that do this, they could also benefit from this kind of hack.