I doubt there will be a new FF MMO any time soon because...
- They have to establish that the current one cannot be salvaged, like happened to FF11 early on in its life.
- FF11 was made for consoles and didn't exactly compete with newer MMOs well, or emerging specs.
- It takes years to make. It took 5 years to make FFXIV (which failed and then was remade in 2 years by reusing assets and lore).
So they would need determine that FFXIV cannot be salvaged or upgraded, which is not the case because we have a graphics upgrade incoming, we have had network and sound improvements recently, the FF16 engine was hinted to have been based on FFXIV's, a rough 10-year-roadmap has been provided by Yoshi-P and even if all that were not so, it would take at least 5 years to make this new MMO because that is how long it took to make FFXIV and it took even longer to make FF16.
Now you could say "but maybe they
have been working on one for 5 years already". But there is no evidence of that. It would almost certainly be CBU3 that developed it, and they have been stretched between two games already, and then there are all the previously-mentioned points such as how FFXIV has a 10-year roadmap and it has also been growing each expansion so far.
Could Square Enix change the combat in FFXIV to use action combat?
Sadly no, because if they change a battle system that a lot of people are used to or appreciate for various reasons or are used to from other MMORPGs, many of them will probably just quit and I doubt SE will risk that.
But there could be some middleground, such as a specific class that plays similarly and boss fights that use the FFXIV style to resemble FF16 a bit more (ie. telegraphing with animations instead of standing there casting or showing red circles). Like Giruveganaus, Rathalos or Hrodric Poisontongue.