Well, sequals suck in as good as every case and degrade the earlier parts. FF is the name of a collection of games that are actually NOT connected, so FFVIII is not a sequal for instance.

A safer way to make sequals if to make chapters, this means that you will have to plan on making more parts and leave room for more chapters, I mean that someone in one chapter has still got some ??? which can be the basis for the next chapter.

This way you don't have sequals, but you have parts. The downside of this is that you have to say there will be more parts, when you come with the first, even if the first doesn't work out well.