The issue is mainly things like quality of life. So what would be better, if they did anything like this at all, is:
  • Simulate the combat system from prior expansions. They can already have a separate leveling system, so it could technically be a separate leveling system that you can turn on, and then that uses old battle system data and functions much like how Eureka or Palace of the Dead have separate leveling systems and adjust how battle values scale.
  • Restore access to old versions of attacks and traits when enabled. I know this is possible because of the fact Blue Mage can enable/disable attacks into the Action List.
  • The "instanced area" with this different combat just happen to resemble all the open world and dungeon and raid content from that expansion.
By far the most controversial thing in every MMORPG is changing the combat. It is the biggest risk any MMORPG takes, and predictably so after decades of MMORPGs.

But anyway, I think a better and similar solution would just be some sort of skill tree system which they will probably elaborate at Fan Fest later this month. Because then you could simply choose current gameplay vs old gameplay depending on playstyle.

The only other reason to do this than the combat system is to make old content relevant again, but they could do this easily by adding Faux Hollows for doing a random Extreme trial MINE each day, and for doing a random set of 4 Savage raids MINE each week. Oh, and setting all previous expansion dungeons and trials to MINE by default.