I'd say the most important thing would be to get the first section of the game done completely.
If you get, say, everything up to Cloud meeting Aeris done, or even everything from the Reactor 1 mission done, you'll be able to play a significant amount of the game with entirely high-res backgrounds and maybe even release a kind of "demo" patch. This will give a greater sense of progress, and therefore more motivation to complete the job, than just doing random scenes (or the most common scenes) from all over the game in no order and having nothing playable until the job is near completion. Since this project will take a very long time and involves remaking hundreds of scenes, motivation will be a big issue sooner or later.