The game can already sense whether or not the player is "In Combat" -- Road Bonus does not work while the player is in battle with any mob (even if a mob merely aggro'd).

So the detection of "Combat" or whether or not a mob has aggro on you is already in-game right now, right?

Is there a reason we couldn't have some sort of indicator on-screen (of course, give us an option to turn it off should the player not desire to see it)?

Currently, in game right now, the only way to tell if something aggro'd you, is whether or not you got road bonus. This is all fine and dandy, if you're actually on a road which you aren't always on a road.

Sometimes, if I'm walking across an open plain, it is nice to know when something that just popped behind me is chasing me to attack me. There's currently no indicator until I actually get hit with an attack that something is attacking me. It'd be nice if I had more of a prior warning than that.

Also, it is nice to know when I exit combat. If I'm running away from a battle, it is nice to know when it is safe to stop and cast a heal, and turn around to prepare another pull attempt.