Everybody within X distance enters combat even if stealthed = fairly simple. This is already essentially implemented, just for non-stealthed characters. This could include a surprise round if the entire party is undetected when combat begins.
Enemies use detection circles instead of lines = moderately simple. I assume that Larian already has the ability to easily adjust sight-cone width; just set it to 360 degrees.
Enemies use both a sight cone and hearing circle = more difficult. However, it is still using mechanics that are already implemented in game; I'm not sure if the code currently allows two detection areas...
Attacking from stealth removes stealth = extremely trivial
Enemy AI is better at detecting/approaching stealthed characters = more difficult. There are a variety of ways this can be implemented, with a simple implementation being: "if the enemy can't detect any PCs, they walk towards the last location they saw one. Otherwise, they take the Dodge action."
