Most likely, and probably 6th and 7th... necrotic runes.
If we follow the evolution of the Magic combat style, we should see different schools of magic like Ancient and Lunar. They just didn't come with Standard Magic at once. Likewise, we should see different schools of Necromancy and necrotic runes too later... and perhaps we will discover different necrotic rune altars first.
I think it's very possible for sure. Maybe something like 'Plague' (maybe that's a bit too close to Miasma?) or they could go down the route of the apocalypse theme ('War'/'Wrath' -- this would also match with OSRS, 'Famine', 'Pestilence')
I could see an argument made for a Wrath rune that gets unlocked with the quest where we (presumably) fight Vorkath -- this would also match with OSRS's Dragon Slayer II unlocking Wrath runes. I wonder what you could do with this 5th rune though. I think there are lots of avenues that could open up depending on what new incantations they unlock.
@Chee__RS
Maxed
Ironwoman
Completionist
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Max: 28 Jan 2018 | MQC: 9 Aug 2019 | Comp: 18 Oct 2019
RS3 doesn't add items/NPCs for no reason and not related to lore 99% of the time.
We are going to have new necrotic runes for new incarnations and a lot of new content, but more than likely we will hear the stories with a lot of item dependencies before we actually see them in the game.