There is no way to simply transform money into more. Do herby runs to develop profitable skills, such as hunter, fletch yew longs, anything similar. Someone said 99 fishing, I'd say that's probably it, just make sure you catch rocktail between RS gold 90 and 99. Sure, you can however, it's slow and a long road depending on the location you start and how committed you are you could turn 1gp into 1B in a matter of minutes.
The way I'd approach it even though you've 15 million is to invest in something that oscillates between a high and low price , but is held in the middle due to its alch value. At the moment, you shouldn't to be taking big risks when you invest your money but you should build it slowly. 15M , which is a reasonable amount will gain you about 400k profit on each period of the green chaps as an instance.
As they don't fall below 2200 for very long because they have an alch value of 2380 you could buy them for 2200-2250, and sell for 2300-2350. It is not the fastest method but it's consistent. even if you're a member you can go to cheap OSRS GP more exotic products like black chaps. They are available for purchase at 5400, and selling them at 6000+ , but if don't time it right, you could end up with stocks for quite a while.