Why not speculate about number and aspect of the Old Gods? Following my logic, there are 5 of them on Azeroth.
The first reason is - being a sort of parasites, each Old God needs space to feast, to drain some part of planet. It is known that some of the, like Yogg-Saron, have deeped their "roots" deep and far enough into the earth so that their deastruction would inflict irreversible damage to the planet.
My point is - perhaps, the landmasses are divided between them, like sort of domains, kingdoms? We almost have a compete shape here, look:
1. North - Northrend - Yogg'Saron.
2. South - Pandaria - Y'Sharaj.
3. West - Kalimdor - C'Thun.
4. East - Eastern Kingdoms - ???
5. Center - Maelstorm - N'Zoth.
So, is there any evidence of another Old God still missing? We'll see.
My second point of the topic is about their aspects. Look, what if the Titans gave blessings to the Aspects not randomly, but in order to coutnerbalance respective Old Gods' chaotic powers. That would mean:
1. Alextrasza - Aspect of Life vs Yogg'Saron - God of Death.
2. Netharion - Aspect of Earth (order, stability) vs C'Thun - God of Chaos.
3. Ysera - Aspect of Dream - vs N'Zoth - God of Nightmare.
4. Malygos - Aspect of Magic vs Y'Sharaj - God of Sins (magic is known to corrupt its users' souls, rendering them vulnerable to pride, envy, violence etc).
5. Nozdormu - Aspect of Time vs ??? - God of Fate (e.g)
I clearly understand that this theory is highly speculative, for it is never mentioned about this charge given to Aspects, just to secure this forces, not a word about struggling with Old Gods, we don't even know whether the Aspects where aware of the OG exsistence at the moment of their appointment. But, there's nothing that can firmly exclude this version, especially considering clear opposition between at least 3 Aspect - OG pairs. Malygos - Y'Sharaj is less clear, for there's no such firm opposition between magic and sins, and moreover Y'Sharaj was destroyed by the time...
Still, there's one hint about the existence of one more Old God in the Eastern Kingdoms, opposing Nozdormu. Those who've read War of the Ancients trilory might've noticed Nozdormu's remark that only one creature could interfere with timelines with force superior to his own, but its name is too dreaded even for him to be spoken.
My guess that his enemy is god of grim Fate, in opposition to time with multi-optional ways that gives us a free will, Fate seals us merely puppets with no will, so Nozdormu's work might've been to protect the world from inevitable Fate (which is Hour of Twilight), unless to the point he either surrenders to fate, or becoming ready for any means to prevent this fate to come to pass.
Indirect prove might be that just like behind Xavius and the Nightmare was N'Zoth, this unnamed god might be inspirator of Infinite Dragonflight.
So, here is my theory:
1. Yogg'Saron - god of Death in Northrend - vs Alextrasza, aspect of Life.
2. Y'Sharaj - god of Sins in Pandaria - vs Malygos, aspect of Magic.
3. C'Thun - god of Chaos in Kalimdor - vs Neltharion, aspect of Earth.
4. ??? - god of Fate in Eastern Kingdoms - vs Nozdormu, aspect of Time.
5. N'Zoth - god of Nightmare in Maelstorm - vs Ysera, aspect of Dream.
P.S. - if some of you guys has WC 3, you might find a custom campaign for it - "a Dwarven Campaign", or just a single maps it consists of - "Defenders of dwarvekind", "City of the seven mithril golems" and "Temple of the Old gods". Second map is devoted to a dwarf expedition, a dig site deep beneath the mountains of Khaz'Modan, where a long forgotten ancient city of dwarves resides - the Temple of the city was actually a prison of an Old god, like Ulduar, guarded by the said Mithril golems. The story is really interesting. I wish Blizzard would ever make a raid on its motives...