Final Fantasy I: A dungeon and dragons inspired turn based RPG hold together by hopes and dreams. Hardly any story, absolutely no character development (in terms of story). Basic, but decent for the time of release.
Final Fantasy II: Realized they had people who can actually come up with some of their own monster designs. Completely threw away the old progression system in favor of...something...different. Established a lot of the now recognized Final Fantasy tropes like certain recurring characters and items. Tried some character writing.
Final Fantasy III: Got rid of most of the remaining D&D stuff. Again changed the progression completely to a job system. Bit half baked and very grindy, but decent for the time. Character writer seemed to be on holiday for that one though.
Final Fantasy IV: New combat system. New focus on story. Character writer had one hell of a holiday and wrote a huge script. They then threw away 80% of it because the wasnt enough space on the cartridge.
Final Fantasy V: Again...new progression system, similar to III but on steroids. Also it just has a completely different vibe from any of the other games.
Final Fantasy VI: Basically a greatest hits album, writing staff got a raise. Went all out. Changed progression system again.
Final Fantasy VII: Unrecognizable as a Final Fantasy game. Changed everything except for the general combat system. Did introduce a lot of new stuff in the progression system though.
Final Fantasy VIII: Again, unrecognizable as a Final Fantasy game. Being different then what came before cemented itself as part of the series by now. I mean have you seen the progression system in this game?
Final Fantasy IX: Greatest hit album part II. Wanted to go back to the roots of the series for a bit to please old fans. Didnt sell as well as VII and VIII though.
Final Fantasy X: New combat and progression system (again, one might feel like there is a pattern to this), though still turn based. Again very atypical setting for an FF game. Ditched world maps.
Final Fantasy XI: Online...that was a change
Final Fantasy XII: Kinda like XI, but offline. Completely new combat system...again. Very different kind of story for being within the main series. Also, hunts.
Final Fantasy XIII. New combat system...jesus stop it already. Introduction of one of the worst characters in video game history (imho). But lets not lose HOPE. No, actually, lets.
Final Fantasy XIV: Bad until it was one of the best.
Final Fantasy XV: New combat system. Only one permanent party. Bromance. Lots of potential but capitalism seemed to happen somewhere along the line.
Final Fantasy VIIR: New...combat...system... fml.
What combat system are we nostalgic for exactly?
Like the true turn based one, the ATB turn based ones, CTB, gambits, whatever XIII was doing? I'm so lost. Seems like changing stuff, going with the times and adapting to new trends is kinda what made this series stay relevant for 30 fucking years in the first place.
It might seem like they changed combat systems more often towards the second half, but remember that in the early days there was a new FF game almost every year, so the industry didn't change as much between games. Later on the gaps became much bigger and therefore the changes between games grew.