FF12 is godlike overall.

What strategy is there in the older battle systems, though? FF has always been piss-easy after the NES titles, a feature that I feel Yoshi-P captured perfectly in 16 :D

12 was highly underrated.  In terms of strategy, combinations of materia in the original was really cool.  10 had very unique characters with different skills.  

What different skills did FFX characters have beside Limit Breaks?

Only Yuna could summon.  Auron was heavy attacks.  Tidus was speed enemies.  Riku was good against machina.  I forgot the name but there was a dedicated black mage.  

Wakka was flying enemies.  

Isn't this just different stats apart from Yuna's summons? Wakka can hit long range enemies, but if I recall correctly all characters can do the same after unlocking certain skills (or was it just improving their accuracy?). I maxed out all FFX characters, but it's been ages since I last played the game. Still my favorite turnbased gameplay system to date.

I think in the HD version the sphere upgrade system was changed to allow for custom builds.  I don't think in the original there was as much freedom.  I could be remembering wrong.

You can only gain access to other party members' sphere grids through endgame Lv. 4 Key spheres, otherwise, they remain locked in their niche.

In the HD version, you can do the Expert grid that allows that sort of freedom from the beginning, though.

FF12 has some unqiue elements. The micromanagement of Gambits and the customization was truely epic. The thing with FF12 was that I think you could customize your Gamits to play the game without you interacting much. Thats why I much prefer FF7 Rebirth's combat.

... But the dream for me to see Sqaure Enix develop a game in something like FF12 but with the combat engine of FF7 Rebirth.

I played the original version (NTSC) and dropped it after putting some 120 hours into it. Then replayed the game about a year later as the "International version" which had the expert grid, Dark Aeons and Penance, etc. International was the version where I maxed out my characters but I didn't touch the expert grid. I do remember using lots of Lv.4 keyspheres, and filling up all the empty nodes.

It's cool that a stripped down version of Gambits made it into a Rebirth minigame. Who knows... they might expand and utilize it in the main combat in the sequel. I share your complaint about FF12 Gambits... I think there gotta be a penalty of sorts assigned to each Gambit (costing more MP, slowing down ATB, lowering stats, cooldowns, etc). Or maybe just make it limited or only apply these penalties to hard mode.

Just gonna add my 2 cents that FF12 was always a masterpiece.
10/10, has always been. Being my favorite over 9.
Did not play 15 or 16 yet, tho.