Problem is, some of those systems were abandoned for a reason. Maybe they just didn't work, or were replaced by a newer one, or had a purpose that's been rendered moot 10 years later, etc.

Chocobo companions - Sure, they could increase the level cap, but to what end? The open world is already easy enough that it's not necessary in the first place.

GC ranks - Same thing. Yeah, they could add more, but why? What purpose would it serve?
(IIRC, the last time this was asked in an interview the answer was something along the lines of "since GCs don't really do anything right now there's no point in adding new ranks until we figure out where to go with them.")

Hunting logs - Replaced by the daily/weekly hunt system, which lets it be ongoing content rather than a one-and-done thing.

Squadrons - Probably always just supposed to be a prototype for Trusts and duty support.

Guildhests - Designed from the start to teach players about mechanics as they level up. There's no real reason to make more, especially since over time the game has gotten much better about teaching mechanics in regular dungeons and raids as you level.

And so on, and so forth. Most dead systems are dead for a reason and don't need to make a comeback int he first place.