As an old time Final Fantasy player, I think others might enjoy this concept.
Similar to the old Final Fantasies, you would run around the open world to trigger random encounters which would teleport you into a phased battle.
You could do so alone, or in a group.
Based on zones and location the difficulty along with level would be increased of the random encounter mobs.
(you could take fishing holes as an example for how this would work)
This system would also allow for mobs to be available only at certain times of the day, along with weather requirements.
The mobs inside of these random encounters would be of "increased intellegence"
They would have more mechanics, similar to a primal for example. But possibly not as much Damage.
(although it would be interesting to have some extreme difficulty ones as well, for endgame. Possibly weapon fights for example. I.E. Omega weapon, Ruby weapon etc.)
In return to this phased area fight and increased difficulty, the rewards for leveling would be much higher, along with increased drops on items for crafting.
Random Encounters would have a Daily limit on them similar to Levequests, so that it can not be exploited. But would not accumulate, So they would have a daily allowance.
This system would relieve some of the grind in the leveling system currently in game.
(this is just a rough idea)
EDIT 1: There could be a new hunting log created specificly for Random encounters. With achievements depending on the mobs that you slay. Possible minion/mount rewards of some of the notorious/iconical monsters which could be included in the system.
EDIT 2: it's all pretty much in the game already as it is. The fishing system, and classquest/story quest fight system. (just remove the purple circles or NPC entry dialog.)
They don't have a CHANCE to spawn within a time frame, it's that their encounter is available even multiple times at certain time of day and also weather combinations.
EDIT 3: It would be an optional system, for example something you could trigger in the UI to enable random encounters.
Not just permenantly wandering the world you might randomly encounter a special monsters.
Thats the entire point of the system, to create that nostalgia effect.
Along with having other ways to level/get achievements etc...