I'll keep this short since I talk about it so much as it is.
I like when the enemies around and the weather situation change the battle, this is Macro-environment for me.
https://forums.ashesofcreation.com/discussion/52939/lets-theoryraid-1-jormungand-vs-xenojiiva
Xeno'jiiva's arena in its 'normal' form also has some elevation changes that you can use until it destroys them, but it's subverted because the thing you normally use elevation for (mounting the monster) doesn't work on it.
I like when bosses can 'call adds' but it isn't just some base mechanic they have at a scripted time. Tumok shaking spiders out of trees would be in my wheelhouse then. Muh immersion.
https://forums.ashesofcreation.com/discussion/55622/lets-theoryraid-2-cyclops-v2-no-depth-perception
I also like chaotic/war settings, especially when it temporarily 'takes over' a familiar location, causing the player to rethink the area and form new 'memories' and concepts.
https://forums.ashesofcreation.com/discussion/55673/lets-theoryraid-3-putting-heads-together-hydra-illuyankas-ffxi-vs-tiamat-onigiri
Illuyankas can be part of Besieged and attack Al Zahbi, alongside many troops, and even though this is the equivalent of a Castle Siege defense so there are many players to deal with the many enemies, it still feels good.
In terms of actual environments, everything about Toublek is great, and it becomes greater when you fight the Tier 2 version Kertaki. FF11 would have added even more to this, making the 'weather' in the area change based on certain area-statuses like the fog, or having things like 'windy weather' affect the fight.
https://forums.ashesofcreation.com/discussion/58812/lets-theoryraid-4-briareus-and-tourblek-reimagined
https://forums.ashesofcreation.com/discussion/60640/lets-theoryraid-5-tiamat-ffxi-vs-firebrand
FF11 Tiamat is mostly, again, just 'enemies around and weather situation'. Firebrand, when fully implemented, will be even more amazing for similar reasons. Steam to block vision, ground hazard lingering burning areas, locations where the breath would be blocked by terrain... I look forward to seeing Firebrand fights in all sorts of cool environs.
Other than that, as another poster mentioned, the 'ground falling out from under you' thing is always at least somewhat interesting (FF11 Diabolos fight, for me, but obv many others).
There are too many recent examples from Throne and Liberty to even go into. They seemingly try to keep these complex mechanics out of Field Bosses since those 'need' to be accessible to the least invested players, but I'd say that there are more battles
with
environmental components than without them, so it would really just be 'a list of TL Dungeon Boss and miniBoss mechanics, excluding Heliber/Karnix'.
My personal favorite is Carmine Rage Island because the layout is just so good, it really gives that 'I'm having an adventure in a goblin forest' feeling, MiniBoss #1 being a 6v6 is fun but not really environmental. MiniBoss 2 has an environmental 'mechanic' placed around a Goblin... Hellcaster (idk we just call this mob 'Hydrolox').
Hydrolox has various moments where you can imagine the Arena floods with magical energy from some huge Runestones that surround it, and each player gets one protective rune 'attunement' applied to them. To survive the blast that will come or at least not get massively chunked down health-wise, you have to 'notice which Runestone has the mark matching your resonance, and move to it'. But the Runestones themselves aren't all in your field of view and the mark doesn't last long. Similarly, a teamwork aspect comes up since if you miss yours, you can 'know not to go to a spot where you see a teammate'. Hydrolox also changes position and focus during this, I think, and being aware of all three things at the same time enhances the feeling of stress in the boss fight.
There are also multiple hints on the ground patterns of boss arenas, which I'm not personally a huge fan of (and my group has to constantly be reminded to take a look at the floor before engaging the boss to get a starter-guess of where to stand). I think it was also for 'accessibility', but probably for communication purposes, as it makes it easier for the Lead Adventurer to coordinate the.. "C-Ranks", for lack of a better term.
The inverse can be 'seen' in Monster Hunter games (well, moreso World), where the spot you fight a monster changes the planning and fight, sometimes massively, and environmental 'bonuses'/hazards abound. Since the monster moves around or can start in different areas on the map, it helps somewhat, as even 'special hunts' aren't really 'scripted' in the normal sense. Things like 'Kushala Daora going into locations it wouldn't normally go and creating new battle mechanics' like its Fire tornadoes.
MMOs are often forced to limit this type of content for the sake of not disrupting newer players, which I understand (but of course, as a 'veteran', find to be a little disappointing). There are many arenas and environments that can feel recontextualized by adding boss encounters in this way.
FF11 eventually implemented this using a system that didn't have the best immersive effect on me, the 'Voidwalker Notorious Monster'. This was done in the way it was mostly to prevent low level players from stumbling across potentially-aggressive forms of 'things they absolutely should not mess with' in a punishing game, but the ability to fight a devastating Raid Level black dragon in what has, for all your 'life' on that character, been either a Starter zone, or at worst, a war battlefield in one small section, makes an impression very different from fighting effectively the same thing in just a 'standard arena', though that Arena had good 'vibes' too.
Long live Anguis, Long live Yilbegan, down with Overlord Bakgodek.