I would like to see a revamped battle system with more emphasis on skill and less on automated actions.
In FFXI, you just engage and auto-swing.
I think it would be better in FFXIV if, for example, DD had to manually target and there was some element of skill to how hard you hit the mob.
Tanks had to manually block with their shields. MNKs could manually counter or guard. Ranged attackers had to concentrate and target etc.
I know that lag could play a part in fu*cking things up so it couldn't be too complicated but then lag ***** up all online games in some way or another - you just have to deal with it.
I definitely think it would make parties more interesting if you had a real element of skill in it and would make the phrase 'skill is greater than race' meaningful.