Kind Monsters - With a grudge: Lets say Phurbles normally heal people, but if they are murdered in mass numbers they at first, stop healing you depending on how many you killed, and eventually start to agro when you go on a mass murder spree. I'd generally say keep it individual per character as if it was a server based behaviour I'm sure they would try to comit genocide on us withing a hour.
Make it like the Pixies from XI (yes I'm readying the coffins of those who find XI a sin against god), they'll be less inclined to heal you or fade from existence (in the present in XI's example). Same with the Fomors, they're generally harmless, but if your hate spikes to max they'll aggro you and even become true sound at a point as well as enhanced abilities, e.g their defense down move will eventually drop your defense to 0.

Hidden Monsters: Long are the days I have waited for a Antlion to come up from the sand and try to snatch me up whole, or where I had a slight heart attack when a Chigoe's name would appear amidst thin air I thought was empty moments ago.
So much this. You're walking in a new zone, no knowledge, bam, you get amushed by an antlion and sometimes oneshotted if it critical and you're low enough level. This is what disappointed me most about antlions in XIV, because they're generally seen as..well, not so surface dwellers. Only the "workers" were ever seen above ground, but even then some were underground waiting on eat you.

"Smart Linking" Has been brought up by the developers that some monsters will Call for help if you are too strong. I would love to hear more about how this will work!
Yeah the way I envision this is like if you ever fought this monster in XI, Almethia I think it was? She'll call all Rams and sheep in the zone to help in the fight. So something like that would be interesting if you're suddenly swarmed by enemies (if fighting swarm type enemies, like Gnats.)