Hmm, the recent post by Mike Z in the execution requirements thread made me think.
Usually in most FGs, once a combo starts, the rest of it is just a series of inputs with specific and precise timing. You could program a macro to do it if you wanted. There are particular combos in particular games which are exceptions of course (eg. loop juggle combos where you have to adjust for opponent altitude / reaching the corner).
So yeah, what about a FG where some properties of the combo system are not predictable and have to be judged by eye?
eg. launcher attack hits opponent up to a variable height. You can always follow up, but the timing will vary and you have to judge it visually.
eg. different air juggle attacks can raise or lower the opponents altitude. The opponent also is hit up or down by a variable amount, so to prevent him from falling out of your combo you have to watch his height and choose the correct attack.
eg. a ground combo that involves a knockdown and then guaranteed oki attack. But the period of time the opponent stays down will vary, so you have to hit the oki based on watching the wakeup animation.
Putting aside my examples, I wonder if there have been any obscure FGs in the past that have such a design? (ie. all combos have to be judged by eye, and are impossible to macro.)