Something to keep in mind that the only thing connecting most FFs are, lets see..

moogles
chocobos
names of magic spells
names of summon monsters
and some key enemies like Bombs, Behemoths, etc.

Honestly, if they chose to they could go on forever making games under the Final Fantasy name, and lord knows they will unless the sales drop off.

Whether it be FFIIIj or FFIX, summoning Ramuh or Ifrit and casting Firaga and Flare will never get old