Combo Action Button

[Reynhart]

Fel Cleave and Inner Beast are both on the GCD.

Well, yes, that's what I said. They don't share a real "cooldown", allowing to use one while the other is "on CD".

If you're referring to Recast time however, that can easily be done via a secondary number in the top corner of the button denoting the cooldown of the other skill.

Yes, it could. But that's a "new" feature. Until now, the suggestions never asked for anything new to be added into the game, nor potentially removed any information to the player.
That's why I said it could be "tricky", not that it couldn't be done.

Of course, they could go the Bloodletter/RainOfDeath route and decide that, comes SH, those two would share a CD, but that's a completely different point.