First of all : I think every creatures should have 2 detections area.
- The first one is the visual one. The cone we actually have but any creatures should see a lot more further ahead. It could be very good if the further is the hidden character, the more the DC to see him is difficult.
- The second one should be the sound. A circle all arround the character would do the job if it's not possible to add real "noise" detection.
Like yourself I didn’t play much with stealth, even through I played with rogue intending to test stealth. Advantage and sneak attack is far quicker to get through other means, and with jump and shove using bonus action to go stealthy kinda feels like a waste of action.
I like your suggestion (reminds me more or less of what Deadfire did - and different creatures could have different sound/vision cone depending on their hearing/sight). Walking through hearing cone might require a stealth check.
I can see how bonus action stealth can be problematic as it makes enemies unaware of our position - one could potentially take potshots and hide without AI doing anything, if that is how it works. Making hide full action would mean that unless you are a rogue, you can attack and hide on the same turn.
What OP suggests sounds like a solid idea as well - enemies “investigating” last known position, especially if there is no other target. There even could be units with “Hunter” mindset - enemies like rogues might prioritise looking for stealthy characters, over charging at full-plate armoured warriors.
