Coding a 2D JRPG?

I don't know anything about C game or gui libraries, I'll figure it out as I need to. I don't want to use a very complicated library, but I do want something that is maintained, Linux native, and portable to at least Windows 7.

SDL 2 is cross-platform and vastly used for I/O, sound and graphics.

I would reuse parts of the modules as much as possible, with sub modules you can switch out for thing unique to each game. Thus you could hack the FF5 job system to FF6 with far less effort than it would take for assembly language. Same deal with extending the scripting systems, upscaling art assets, improving music, and so on.

This is a good approach.

So ideally you could use the existing FF rom hacking tools and export your changes (or rip soneone elses rom hack), you could rip it all out and start from scratch using PC specific tools like GIMP/Audacity (the scripting would just be plain text), or both.

They all not all open source sadly so some impossible to modify. I would personally not use the ROM as a base because everything would need to be coded in the ROM specific formats for data and modification and you would need to code an export format for everything that is more suitable for your game. For my project I was thinking just coding an editor for data and scripting languages and use just one format for each thing. Using a ROM as a base seems to doubling the workload to create editors. Maybe some export features for graphics but that would be it as far as using the ROM would be.