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.

thats epic good idea i agree 100% nice thinking jp.

Yes please

ZMI altar.

http://runescape.wikia.com/wiki/Ourania_Runecrafting_Altar

ZMI altar.

http://runescape.wikia.com/wiki/Ourania_Runecrafting_Altar

It's used for soul runes already.

I disagree with idea. Thanks for suggesting.

Would benefit both PKers and PvMers alike since the saved inventory spaces are nice. Also would add a reason to train runecrafting and a proper alternative to the rune pouch that bunch of ppl have suggested.

I think that mind and body runes should also be required for the catalytic rune though.

I really like that idea. However I don't think it's really going to save you inventory slots, because you will end up using 2 inventory slots for spell casts anyway with that system. I do however see this as being really convenient, because you could cast every spell in the game with just 3 inventory slots.

Maybe the combinations need to be adjusted or be more creative? For instance we could create a rune that allows casting all offensive spells in a certain spellbook. So we could have a wave rune and an ancient rune that would allow you to cast a fire wave or ice barrage respectively.

Such runes will definitely be untradeable and could even be crafted from pure essence instead of rune essence. What we could also do is add a rune pouch (adding multiple runes into 1 inventory slot) that is craftable with 90+ Runecrafting and degrades over time.

Thoughts?

What we could also do is add a rune pouch (adding multiple runes into 1 inventory slot) that is craftable with 90+ Runecrafting and degrades over time.

Rune pouch would be a far better idea than this and it would be a lot handier as well. Could add 3 runes of your choice in there. It would also fit in PvP store perfectly instead of it being craftable.
Maybe make it even wearable and give it small bonus' like offensive magic and prayer.

Same could be done to bolts but instead you could only hold one type of bolts in there but the quiver/pouch itself would give small offensive ranged and prayer bonus. Would also be obtainable with blood money from PvP store.
http://oldschoolrunescape.wikia.com/wiki/Bolt_pouch & http://runescape.wikia.com/wiki/Bolt_pouch

Astral altar. Not being used at the moment. Also with a bank nearby. Would bring life to lunar isle since the teleport is available.