We already have it for PvP.
Hien has it when you play as him for that quest in Stormblood.
An option to engage Combo's for us PCs would be real nice out in ordinary gameplay too.
Of course people are gonna say "Git Gud", "Learn to Play" or "That's overpowered" or something. Ok then so we hinder it. If you press the button while the GCD is on then it resets the combo and you start over. So you have to time your button presses carefully. There, no button mashing. And due to human reaction time, it's not perfectly optimal for high end play. Although high end play is still possible with the set up.
It's not auto-mating combat or botting anything out at all. Just instead of the same 1, 2, 3, 1, 2, 3. It's just 1, 1, 1. instead.Not counting weaving your OGCDs in there.
And as a bonus it helps lighten the intense button load of classes like SAM or MNK.