There are some "triggerable" encounters in the resource areas with lots of potential.
Like the shaman hut and altar in Purple Lotus, the harbinger in Poitan and the totem in Laceish. They would be perfect for content in between solo, groups and raids.
Also these zones use a "random" encounter table already...now imagine this a bit expanded with a unique loot list (like purple) too. There could be a counter increasing the chance of spawning a boss, depending on how many minions have been slain for example (like an overly jealous prospector boss)...
Expanding is mostly a better route than removing or replacing content and easier than adding completely new stuff that might seem out of place (amphi fire demon boss for example).