Tier based bossing
How does this work you ask? Well the layout of the idea is pretty simple and unique as this is something I've never seen any modern rsps do. You get a new boss and place him somewhere somewhat accessable. You give this boss some unique restriction like ( Total level must be above 1750 to attack this boss).
You add some cool drops to this boss but you also add a key drop to him on a rare 1% chance. This key will grant you access to the next boss's lair. If you try to access the lair without the key you'd get a message saying "You cannot enter since you don't hold the key."
The second boss would hold powerful items at a low drop rate. You can keep expanding this by simply adding a different key drop and another boss.
Keys:
The Keys would be untradable/undroppable/un-loseable.
Schematics:
Location of first boss:
The first boss shouldn't be really hard to kill and a pretty fast respawn rate (30-40 seconds).
Location of next lair:
Add the restriction to the door and add a trap door that leads you to the next bosses lair.
Location of next boss:
This boss should be very powerful with a respawn rate of 250-300 seconds.