Basically the idea and point of this update would be to introduce "overloads" of runecrafting. (Untradeable and strong item to be used in pvm/pvp/skilling.)
This would also introduce a new and fast way to train runecrafting past level 99.
Catalytic rune and
elemental rune would act as elemental runes and catalytic runes as their names suggest.
Elemental rune would act as air, earth, water and fire rune. Catalytic rune would act as chaos, death, blood and soul rune. Catalytic is slightly different than in Runescape's wiki but I honestly don't think all the runes would be needed or are even necessary. Maybe a third custom rune could be made that contains law, cosmic, nature and astral rune? Name could be "skilling rune".
To make these runes you would require 1 of each rune the combination rune contains plus a rune essence and 90+ runecrafting level or even 95+.
1 Air, water, fire and earth rune =
2 elemental runes
1 Chaos, death, blood and soul rune =
2 catalytic runes
1 Cosmic, nature, law and astral rune =
2 skilling runes
For the altar itself where you make these I do not have any ideas or suggestions.