From: Flight
If it were me I'd train up Runecrafting to 91 at the Runespan and then just make a nature-crafter and craft Nature runes until you've made your hundreds of millions.
Orrrr, make the rune running script, and make a nature altar standing script separately. Just setup multiple players to be running the ess, and have one stand at the altar making double nats. Technically if you have enough runners going, you could be crafting double nats constantly. Say trading a runner and clicking the altar takes "x" amount of seconds. And say you have runners taking you 27 essence per load (making it possible to run ess fresh off tutorial island), that means you craft 54 natures every "x" seconds. At the current GE price of 299 coins per nat, you'd be making 32,292 geepee every "x" seconds.
Now, lets throw some number in for "x", say 30 seconds to trade and click the altar. You'd be making 64,584 geepee/minute => 3875k/hr.
Now lets say we write amazing scripts in which "x" would be reduced to 20 seconds. That's 5812k/hr.
Throw in a six hour fix for the scripts and let them go for 15 hours a day (unless you're a madman for 24/7). That's 87.18M geepee/day. Two altarstanders going at it for 174.36M /day.
Ha though nobody would ever release these for public use if they made them. This can technically work for any runes, so you can have accounts standing at the most profitable altar and have the runners go there for even moar geepee. I'd propose having the runners go through the abyss though if you're planning to go to multiple altars. And if there is a change of price in runes (too much farming) just change a few lines of code to move on to the next profitable altar until the original recovers/stabilizes in price.
I've just convinced myself to start working on these scripts now. :stirthepot: This may actually even be more profitable on OSRS if your plan is to gold farm (plus I think rune running actually happens on OSRS servers so less chance of everything getting JamFlexed).
EDIT: At the rate of 9720 nats/hr at OSRS price (239 gp per nat), that's 2.32 OSRS gp/hr. Which means 34.84M /day with one accounts, 69.69M/day with two. Currently gold is 1EOC:8OSRS, so that's technically 557.54M EOC geepee/day. So much potential here.