I suggest trying out a few good NES RPGs and evaluate which one has the engine best suited for your endeavor (technologically advanced-ness, similarity to FFVIII/IX, estimated extensibility, ...). Then disassemble and understand the parts that you need, e.g. map/field, battle, sound, etc. This will take a lot of time, but you will learn much about the NES, and with your newly acquired knowledge, hopefully be able to implement a map editor for this engine. If you get this far you can start replacing modules that have to be changed, e.g. battle, character stats, and so on (it would be beneficial to choose an engine that is quite modular; heavily intertwined code is very difficult to work with).
Well, that said, this definitely isn't for the faint of heart, but this is very likely a lot faster than developping an NES RPG engine from scratch. Still, if you aren't very determined and love the NES and assembly code, I'd concur with the others and say you should use a high-level language, or something even higher-level like RPG Maker.
Source: I disassemble games and their engines for fun (mostly SNES though, almost no experience with the NES, so take everything with a grain of salt).