Coding a 2D JRPG?

The format I use for data storage would by necessity be different from that of the rom, both because it's simpler and easier to expand. If it is 1 for 1 with every bit it isn't easy to expand, and it's hard to work with. If it exports scripts to a text file as mnemonics or method calls with parameters as appropriate, it is very easy to expand and work with. The games scripting systems and data layouts become a minimum standard, as I don't see anyone taking a mod from this engine to a rom, but I do see people doing the reverse. There are a lot of good mods out there that change data, scripts, and assets.

Exporting data from a rom is very easy, even if it's compressed or part of a disc image file system. I wouldn't hardcode addresses, instead I would abstract it to data/asset descriptors in CSV files. If a hack or translation or regional difference moves data/assets around, you just locate them then change that entry in the text.

Once you extract data, assets, and scripting from your favorite rom hack/mod, they are already in easy to edit forms. SQL for data, plain text for scripts, png for images, lossless audio for audio, lossless video for video. I'm not sure what the most cutting edge thing for animated soundless images is, are there rplacements for GIF?