Great Idea
You've got a great idea. I loved the Immortals Rules from way back in the day.
I'm thinking about immortals like this. If a character becomes 30th level, then they can chose to become immortal.
There are 5 main 4th edition like powers associated with becoming immortal.
At 1st level Immortal, the character becomes immortal.
At 2nd, if the immortal's body dies, a worshiper can be possessed after a week.
At 3rd level Immortal, the character can resurrect into a physical body. If the body dies, the character must wait a week to resurrect again. The character can still possess a worshiper but the immortal must remain sleeping.
At 4th level, the immortal can command a small army. 100 soldiers per level.
At 5th level Immortal, the character can summon 1 or more immortals of equal or lesser immortal level.
I haven't worked out the details of this yet, but have at it.
My Dungeons & Dragons 4th Edition System/Pathfinder Characters/D&D 1 Castle Amber Module
kira3696.tripod.com/CombatTracker.rar