Not sure if this idea has been suggested already, but I'll just go with it anyway.
Instead of requiring 5 different buttons/hotkeys for each ability in a combo sequence, just have 1 button, and have it change its icon to the next ability in the sequence when you use it.
For example, with dragoon, the chain impulse drive -> disembowel -> chaos thrust -> fang and claw, there are 4 different buttons.
Instead, when you use impulse drive, the icon would become "disembowel" for the period when the combo is still active (i think its like 15 seconds).
So there would just be 1 button you would press 4 times in sequence, instead of 4 different buttons and 4 different hotkeys to keep track of.
Reason I want this is because my action bars are getting pretty heavy duty, and I am running out of convenient hotkey combinations near AWSD
That's all, hopefully it was clear.