Simplest explanation: Elite skill using Runecrafting, magic, and Archaeology. Purpose: Creating and redefining magic and maybe even prayer. This could easily just be a 100-120 magic idea but if wanted, a skill too.
What it would be is essentially breaking down magic, runes, and the existing world around Runescape to learn magic and create your own. Spells are clearly definined. Runes are elements of nature. Examples of what you could do: Break down the fundemantals of blood magic and ice magic and create a unique twisted spell that combines the benefits of both with some disadvantage for balance. If the blood magic healed you 100, and the ice froze them for 10 seconds, now its 75 and 7.5 and so forth.
You learn how to create new teleport spells either player-decided locations or literally create lodestones like dungeoneering anywhere in Runescape for your leisure. High level players have already fought gods, who is to say we cannot begin the path of creating.
This could lead to new prayer books, new magic books, new runecrafting options (new runes and combination runes). Easy implementation would be predefined but unlockable magic book but difficult implemenation would be creating a fully player decided magic book or new spell slots within existing ones that players get to define their own inputs to.
Creating a combined 25% range and 25% melee prayer or something similar.
Again, could just be an expansion on existing but if Elite skills are already planned to be a development on existing skills, there is an idea.